Dear xiao^zhuzhu89,
I think if the guy can leave u for his ex and then leave his ex
again for u, shows tt he is not treating u seriously at all. (Of
cos, it includes his ex). These sort of guys only treat the other
party whos not with him as a spare tyre. He will only think abt u
when hes alone again. So its not worth the effort and all the
things u haf done for him.
I hope ur cut is getting better. I noe it hurts. My advice to u is
to forget him. There are plenty of guys out there who are much
better than him, guys who will ont let u cut ur wrist for them cos
they treasure u and will not let u come to any hurt. (Ty guy u
mentioned let u cut urself and hurt u, so its not worth it.)
Take ur time to forget him. But dun go soft hearted cos it may
cause u more hurt. And cheers up bah! 
U will soon forget the matter and who noes, mayb then u
will get to noe a better guy le! 