I'm sorry to hear that you're having such a hard time. Have you talked to someone about it, like a friend or something? Just the fact of being able to release your feelings and talk about them can sometimes help, even just a little bit.
It's hard yes, but don't give up on yourself. Find things that brings you joy, let yourself be proud over what you manage to accomplish, and don't beat yourself up if you make a mistake. Like you said, you have to look out for yourself, which also means you are the one who can make yourself happy and believe in you. Don't let anyone change who you are and what you can do. You're not her. Be the greater person and rise above that, make better choices than she did.
Here's one thing you can do, for every bad thing you hear, think of two things that you are good at or you have. Or why that's not true. And also, sometimes writing things down helps as well, call it a feelings journal or something. Keep a list of fifteen qualities you have somewhere you see it everyday so you have to look at it, take them in. Another thing... if you feel like it's getting worse, or like it doesn't seem to get any better, maybe try talking to a professional.
Good luck, and remember you're awesome!
