Get help for your mental health issues. It’s not normal to not value yourself and life will be a lot easier if you deal with it now rather than wait 20 years.
Stop being “nice”. It’s ok to dislike people, even without having a concrete reason. You’re allowed not to be friends with them. Even more so when they’ve behaved badly - don’t let them back in to do it a second time, in spite of what your mother tells you.
If a man calls you fat dump him immediately. You’re not, but even if you were he’s showing you he wants to control you by putting you down.
Do the A levels/degree you want to do, not the ones that you think will please your parents. It will probably end better.
Let yourself say no. It’s not liberating to sleep with everyone who asks, it’s the opposite. And when you don’t value yourself it’s actually very damaging.
Friendships evolve. Don’t hang onto them for nostalgia reasons. It’s ok to drift apart.
Learn to love and value yourself.