Don't know what to do?

Hi all.  I've been reading this board all week.  Last weekend I found out my H was having an internet relationship with someone he met playing games online.  She lives out of state.  We argued and he moved out.  I called the girl to ask her how she could do something like this to our family (we have been married 14 yrs and have two children, 11 and 13).  Well she tells me that she was told by my H that he had been separated from me for the past year!  He made up a whole story that he told her.  She apologized several times over telling me she didn't know and would never do that to someone.  She then text me later and told me she broke it off with him.  She even forwarded me the texts he was sending her pleading her not to leave.  We corresponded by text and email a few times because I had lots of questions and she answered them honestly.  Like telling me they had phone sex almost every night.  Well after thinking about it, I didn't want to lose him and break apart our family.  I talked to him about it and told him I want to work things out.  We have been taking each other for granted and lost communication throughout the years.  He told me he wasn't sure and needed to think.  Come to find out, she took him back instead!!  I was shocked!!  I sent her an email to ask her to step aside so my H and I can try and put our family back together.  All she did was send my email to him!  He told me today that he choses her over me and it would never work between us.  He said we argue to much and doesn't think we could ever fix that.  I told him you can't think straight right now because all your thinking about is having a life with her.  He's only been with her for 3 months and it's all over the phone and internet.  I don't understand how he can toss away all our years together for someone he has only been with for a short period of time.  I'm crushed and don't know what to do.  I don't want to lose him.