Firstly i would say you are a very mature person, and i wish i was as wise as you are at 15. As we grow older there are allways certain events which happen in our lives which take us in a new direction, sometimes they are wanted changes sometimes they are not, and other times they are unexpected, suddenly taking us by surprise leaving us wondering how we got to this point in our lives when things used to be so different, which is where i expect you are now.
It sounds to me as thou you were holding onto the past and didnt want to embrace change, it almost seems as thou you were punishing your friend for having new friends by not going to his party. Maybe you didnt like the fact that because he had new friends he wouldnt spend as much time with you anymore. You said at some point he got the impression you werent friends anymore which seems like it wasnt only the party but other events which caused you both to drift apart. When you first met did you used to hang out just you and him? but later on he made new friends and you just wanted to hang out with him and not other people too.
It seems like you really value friendship and that when you make friends with someone you want to be really good friends with them, or as you put it 'form strong relationships'. It seems to me as thou this is the issue as i dont think you would have any problems meeting or talking to new people and i also think people do like you, as you said sometimes people message you from your old church so obviously they do think about you. Deep friendships take along time to develop and dont just happen over night, you cant expect everyone to be your best friend, but that doesnt mean you cant make friends with people. If people didnt want to know you they would just completely ignore you which is not the case, but just because they dont want to spend all their time with you and see other people doesnt mean they like you any less than they did. There is a good saying that goes, if you try and be everything to everyone you will end up being nobody to noone.
You know when you get to college then university the same thing will happen, the old friends you made make new friends and you make new friends as we all go in different directions. I have learnt from experience that when something goes wrong in our lives to learn from it, see the positive side, and move on. It will make you a stronger and wiser person. But if you keep worrying about what you did wrong and what you should have done you will allways be stuck in your past, and if your stuck in the past you cant move on into the future.
It seems to me as thou you have learnt from your mistakes and are ready to move on, you realise that if you dont make new friends you will be alone and dont want that. Just remember not everyone you meet will be your best friend, there are different levels of friendship which all take varying amounts of time to develop.
Dont hate yourself, if you hate yourself how can other people like you when you dont even like yourself.
People do care, if they didnt care they would just ignore you.
Not everyone has a best friend, and those that do took time to develop
You are worthy of friends, just because you may not be the most popular person at school doesnt mean your not worthy of friends.
Dont say that you are GOING to make new friends say you WILL make new friends, by saying going to you are still in your comfort zone and putting it off until later. My advice is dont wait, do it now this summer, go meet your friends at the old church and dont make excuses just do it.