Feeling guilty after sex (also I ruin everything)

A bit of background. I'm a nineteen year old woman. I've known I was gay my entire life. I've always had problems communicating with people because I don't want to come across as weak, among other things.
I started falling for one of my best friends a few months ago, despite knowing she's having a tough time getting over someone she had been hooking up with, who she ended up getting feelings for. We've gotten closer once I made her aware of my feelings. We started hanging out more, and she started holding my hand, kissing me, etc. This happens whenever we see each other. However, I have a problem with overthinking. I think about things to the point where I get paranoid and delusional. This will become prevalent later on in this sad little story.
A couple weeks ago, I paddled her boat (ifyouknowwhatimean). That was the first time I had ever done something with a girl (anyone in general, actually). I'm very afraid of intimacy. It's pretty pathetic. When it happened, I was happy about it. But the day after, I noticed I started feeling a lot of guilt. For what, I don't know. I began regretting it. I'm thinking that it might be because she had been hooking up with a lot of people before her and I "messed around." There lies my problem. It wasn't "messing around" for me. It was very real. And now I'm scared that I might have been just another hook-up for her, despite her constantly telling me that she likes me (but doesn't want a relationship).
She's all I can ever think about. I don't feel comfortable even thinking about being with anyone else, but it kills me that I'm falling hard, and that she can just hook-up with other people. I knew what I was getting myself into, and I'm miserable, but I love her too much to want to move on.
It bothers me that I'm feeling guilty sexually. I have an idea of why, but I'd like to get other people's takes on it. Any advice is welcome. I feel so stuck.

Whit, I can understand your feelings, it is hard moving from a friend realm to more than friends. There is nothing to feel guilty about here either, that I can see; you acted on your feelings. You care for her and that is okay, if she is not looking to be in a relationship, slow it down a bit and let her come to you. I think it is great that she is willing to make an effort to let you know that she does "like you".
It can be hard to transition between the two realms from friends, to lovers or into a deep relationship. My lover that I am with now, has been around the block and I am well aware of her past; she isn't shy about it. We have been friends for a while now and we transitioned to this new level. Do I have trepidation still, yes, however I am aware that when she is with someone she is with just that person.
Communicate with your friend if you can and let her know that you are a bit worried, but if she quells your fears, don't worry about it. I hope that this helps a little bit and keep paddling. Have a great weekend.

It sounds like you wanto be exclusive. Yes? But she doesn't?
Some people have the ability to hook up & move on....w/o the emotional exchange.
What do you want from this relationship? I think u need to answer this first or if more happens it'll lead to more guilt.

Hope that makes sense.