It sounds like you are going through a very painful time and it's really important that you take good care of yourself.  None of what is going on with your ex-husband has anything to do with you.  Yes, he is the father of your children and therefore has an obligation to be a good father to them (as long as he treats them with love and respect.)  But he is no longer available to you.  He married someone else and if they are having problems, that's their business.  He really shouldn't be talking to you about that.

Your boyfriend is probably right that you need some time to grieve, but it's also possible that he's having some cold feet of his own.

I think that he really loves her.

  This is the biggest drawback of getting too close too soon (with or without sex).  A lot of times, one or both of you get over the dreamy, hormone-charged beginning of the relationship and realize - "Hold it, I don't know if I'm ready for this."  Works better if you take it slow and build up.  That way, no one panics because things are "moving too fast."  Why not continue to date him and see how things go?

I'll end where I started - take care of yourself and focus on yourself for a little while.