Your ex-boyfriend is not helping your recovery after this relationship at all.  It is really hard to let go for him and for you, but he is seeing someone else and keeps you around as a crutch to fall back on.  It might not be intentional and he's

But he IS using her.

probably not trying to hurt you or to use you, but you both let it happen because it feels good and right.  Then when his hormones settle he has to tell you that its not gonna go anywhere, that he doesn't want to give it another try, or whatever,

I agree, I've been there, I had to do that.

and I can imagine it breaks your heart all over again.  

Don't let him do it to you.  I am of the belief that after the breakup of a serious relationship the only healthy way to get over it is by not seeing each other at all.

Exactly! End it.

  You need to lay those feelings to rest and live a life of your own.  By the time he comes around again you might not even want it.  But belive me, letting him have sex with you isn't gonna make it any easier for you to get your own life.