I'm so sorry for what you are going through. I certainly understand being annoyed by noisy neighbors, but I cannot begin to comprehend your frustration with this situation.
I have some suggestions, and I really hope that at least one of them will work for you, because you are absolutely right. Your young neighbor needs help, your living environment has become toxic, and you deserve to live peacefully in the space you pay good money to lease. Despite the house of horrors, please know that you are loved, and that your existence is full of beauty.

Suggestions:
1. Soundproofing your Apartment
-It will help dampen the noise you transfer upstairs, making it harder for her to follow you.
-If you have downstairs neighbors, it will help dampen the noise you transfer to them. I don't know if this is the case, but if you have people living below you, this situation might be just as horrible for them.
-It will dampen the noise the girl makes, perhaps not by much, but it might help you get some more uninterrupted time.
-It will give you something to focus your attention on. If you have even a cheap set of headphones(not earbuds, because of the ear infections you suffer from) and an ipod, you might be able to tune out a lot of her noise while occupying yourself with a hammer and soundproofing materials.
-While you will spend some time and money, it is much less expensive than paying a fee for voiding a lease, putting down a deposit and rent for a new apartment, and the expense of moving to a new place.
-It is an opportunity to connect with your boyfriend in an activity that is proactive. It seems that he has been unsympathetic to your frustrations, and while I don't blame you for complaining about this situation, he might be tired of having the same conversation over and over.
-It will cost. Mostly time. I’m not sure how much time you have on your hands, but as it seems you spend a significant amount of time being driven crazy by the noise and trying to figure out ways to make it stop, maybe you could try to reassign some of that time to soundproofing. Depending on what option you choose, how much you have to invest, and how effective you want the soundproofing to be, you could spend a lot of money on prefabricated materials, or you could spend next to nothing. Cardboard is a super sound insulator when it is oriented and fastened correctly. If you are unclear on how to install cardboard sound insulation, message me, and I’ll send you some resources. I’m an architecture student, and I have never, ever had to buy cardboard, but I use it almost every day. Cork is another material that is a great sound reducer, and it will take less cork to do the same job as cardboard, so you could save time and space. But it is pricey, and you mentioned that finances are tight.
-If you have zero construction experience it might take some getting used to the tools you would need to do this job. But if you have ever used a hammer, a drywall knife, or a screwdriver without hurting yourself, I have no doubt you could pick it up quickly. You mentioned you are on disability, so I am not sure how mobile you presently are, but perhaps enlisting help from the bf could help you here.
-It will have an impact on the amount of available space you have. It will take anywhere from 1 to 18 inches off the height of your ceilings.
-It can’t stay in the apartment when you do have the resources to move out. Everything that you put up will have to be removed before you go unless you have consent from the landlord to leave it.

2. Engage your neighbor
You have been living in this apartment for several years now, and I’m sure that you’ve come across this girl before she started harassing you so vehemently. Next time you are sitting at the computer and she stands right above you, knock on whatever you can find and ask, “Hello? Anybody home?” and see if she’ll respond. Try your hardest not to become angry, but remain playful, especially your tone. She has made her life revolve around you. While that is clearly not healthy for either of you, she has fixated on you and it’s not going to get any easier unless you know why. Her social skills might actually be so underdeveloped that she doesn’t realize her actions are distressing you. She may just feel the need to be close to you for whatever reason. If she can’t hear where you are, she might be worried and scared that something has happened to you, or that she won’t have anyone to play with anymore, since her parents are clearly not stimulating enough. She might be dropping things to see if she can nudge you out of hiding, not as a way to make you mad, but much in the same way a kid interrupts me in the middle of a conversation wanting to play. Their world view is not very expansive. If it is all a game to her, change the conditions of the game so that you are in charge of the outcome. Whether you like it or not, you react to her movements just as often as she reacts to yours. Your life has been so dramatically altered that you can’t even go to the bathroom without wondering if she has her ear pressed to the floor. I’m not suggesting that you try and accept her behavior and do your best to ignore it, but rather initiate the connection through the floorboards and direct the conversation that follows.
Have you ever spoken directly to the girl about her behavior? I don’t mean yelling aloud from the floor below to stop stomping. I mean, have you ever had a chance to introduce yourself, sit down together, and ask her what she likes to do for fun, and what is fun about dropping things on the floor?
If you haven’t, I’d strongly suggest doing so. Ask the parents if you can come over and meet their daughter, because you’d like to find a solution to the noise problem. If they refuse, try not to become accusatory, but let them know how hard it has been to sleep, work, and enjoy your beautiful space. That you want to figure out what will work best for everyone, and that you think including their daughter in the discussion could really help to determine a happy solution for everyone. They may still refuse, but don’t give up. If you see them in the hallway, smile, speak politely, and ask again. Don’t give up! A lot of times, how you say something is much more important than what you are saying. It might be, because you mention the family is from another culture, the mother responds to tone more than words. If all she hears is you angrily addressing her daughter, she is less likely to work for common ground. But if she can witness you treating her and her daughter kindly, and patiently, it can go a long way. It might be tough to be cordial, but try looking towards the rewards you will reap once you are all on good terms.
When I have seen children with severe cases of opposition or hyperactivity, I've noticed that negative reinforcement does nothing to deter their actions. Giving in to every demand isn’t an option either. What works for me with kids like this is to engage them for a reasonable amount of time, but with limits set for before and after. Like, “Hannah, I’d love to play space invaders with you. But I can only play one level, because I promised your aunt Maggie I would help her in the kitchen. Let me finish talking to uncle Ted here, and I’ll be there in 6 minutes.” I always pick a number that I know is just a little bit longer than I expect something to take, but never a typical 5, 10, or 15. It’s something we here all the time, and so it doesn’t hold much weight. 6 minutes is very concrete, so a child will sit by the clock and watch. If she continues to bother me, I’ll say, “I agreed to play but only as long as I could finish my conversation. If I can’t finish it, I won’t be able to play.” I'm not a health professional of any kind, but I have ADHD and anxiety. My understanding of my own condition has helped me better relate to kids with similar behaviors.

3. Get out of the apartment.
I am not suggesting moving, but spending more time outside of the building doing activities outdoors, with friends, family and anything else you can think of to distance yourself from the source of your stress. I don’t know how much time you are in the apartment, but if you can join a book club, schedule a weekly dinner out with a group of friends, or connect with a community garden, after a while, your neighbor might find something better to do with her time. This is tricky though, because we all know she needs a better way to learn and play, and removing the object of her focus (you) from her life, could backfire. But please understand you are not abandoning her. You are not her toy, and you are not her parent. While she needs structure, you can’t control how a parent raises a kid. All you can do is ask, ask again, report any abuse you think is happening, and then take care of your own well being.

4. Practice Mindfulness and Meditation
I get the sense that you are sick of people telling you to try and ignore her, or just accept that there is nothing that you can do about it. I agree with that feeling of frustration, but those suggestions aren’t invalid. You can choose to accept and ignore, but only if you have the tools necessary to cope with this major stressor. Do some research on mindfulness and meditation. It has helped me enormously in the past few years. I don’t know if you will benefit from practicing this, but at the very least, it has the potential to lessen the physiological effects of stress. This can keep you healthier, because it’s so much easier to become ill when you are under unreal amounts of stress.

I hope that you can find a way to fix this situation. Your well being is too important to let this continue. Let me know if you want some info or tips about soundproofing, and best of luck.