I agree with nice girl, going to the doctor sounds like a good idea. They can refer you to see a therapist, or psychiatrist, maybe both, and both of these roles can be very important in overcoming self harm, or any mental illness.

As for your worry about speaking to doctors, maybe you can write down everything you want to say on a piece of paper so if you get jumbled up, you can simply refer to the paper? (I've done this before, and it's helped...I just made a bullet point list of subjects I wanted to speak about.)

It's great that you're really thinking about getting better, and even though seeing the doctor can be scary, I think this will be a very positive thing for you. Plus, it seems like you have great support with your sister, it's very clear she cares about you a lot.

I hope you'll keep us updated on how you're doing. I'll be keeping you in my thoughts!