Friendships are hard to manage at the best of times so please do not feel like you are alone there. I think though that your friend probably knows you better than you think, and the fact that they have stuck around and are still there with you is a pretty good indicator that they enjoy your company. How would you feel about handing over some trust, so to speak to your friend, and perhaps accepting the friendship entirely and when you are feeling like you are about to second guess her, shut it down with some self talk like "I am worthy and she is my friend".
Self worth and self love and so very very important and I am so glad that you want to focus on this. I think the first thing to do is to try to do some things that you enjoy and that you are good at, so you can see that you have some success and that you are also capable of joy and happiness, things might be singing or writing or baking, anything that might give you some joy. Small wins and small steps are really important as this sort of self awareness and creating new feelings is not easy and it is not quick...but small steps....one at a time..then another one.
I find writing really really helpful, you can tackle it two ways too, you can have a page that you just purge on and get all of the negativity out and all the things you feel and want to say but would never dare...no one will read it and you may never read it again, just get the feelings out...then on the other page you might start to write all the things that are good about you, this I know will be daunting at first and you may only have a few things, but in time if you keep at it you will be able to embrace this as a special time when you find out really lovely things about yourself. You may start with simple things like, I am good at listening, I am a good friend, I love things that are purple, just really positive and happy thoughts.
Being a young person is so very tough and I can hear how much stress and pressure you are feeling and I would love to reach out and give you a big hug. I really feel that you are going to get the hang of this self love thing, you know why...because you want it...and you are worth it.
Another thing I just though of that make for happy thoughts is smells, things that make you feel happy or remind you of happy times, might be ice cream or flowers or perfume...just take some time to stand there and smell and enjoy the happy feeling it is bringing to you.
Big hugs to you soo-ah