hi again soph
I agree with everyone. I've had a few serious boyfriends over the years (and a few not serious ones as in "you cannot be serious!!" and have found the love of my life now.
Each time you break up with someone you beg for them back, they rule your emotions for a while, then when you look back you think "blimey, what a tit, what the hell was I thinking?"
You are so young and relationships you have now will seem like they can never be repeated.
Your ex is a, sorry, can't think of a polite term. He knows he can get you back, he has done before, so he's obviously at a loose end, sees his ex and thinks "aha, she'll do."
You cannot be friends with him. Find some real friends. People who value you for who you are. Men in general find it hard to be friends with girls they've had relationships with. If he needs friends he will find some he can drink beer with and leer at women with. He will not ask his ex girlfriend! And another thing. If men are friends with girls, they will be wondering how they can have sex with them about every five minutes. I know this because men have told me!
Be strong and true to yourself and one day you will meet a lovely man who will make you very happy and not play mind games with you.