Ex-husband has a new Girlfriend, how to feel?

1 month ago my husband left me - saying he was unhappy. Which was odd at the time because we had a really good relationship and were going for IVF in 3months to start our family (I can't get pregnant and we have gone through 2 years of treatment to get to this stage). In December he met someone and started... show more 1 month ago my husband left me - saying he was unhappy. Which was odd at the time because we had a really good relationship and were going for IVF in 3months to start our family (I can't get pregnant and we have gone through 2 years of treatment to get to this stage). In December he met someone and started seeing her constantly - I was quite sick throughout January so he spent most of his time with her, claiming they were just friends.
In February (after our 3year wedding anniversary) he told me he had feelings for this girl (who had become my friend) and that she told him she had feelings too and the only thing stopping them from being together was me. At the time he told me he wasn't leaving me ever and that he would stop talking to her.
But he didn't.
For the next 3 weeks they spoke to each other everyday, but didn't see each other. I went around to see her and she said she would never steal anyones husband and that she didn't even like him in that way.
After he moved out for a week, saying he needed 'space' and that he would be back, I found out that they had been calling each other when I was sleeping and arranging secret meetups.
He never came back after moving out and said nothing could be fixed, which is not true because there were no problems from what I could see, we had everything to look forward to in the future.
So everyone I know thinks that the girl was the push he needed to just leave and not come back.

Well for the first 3 weeks after breaking up he said he didn't see her and has constantly told me that he has made a mistake and she didn't want him anyway and would come back to me after some time had passed. He even has started talking to our mutual friends again because they said as long as the girl is gone then they can be friends.

A few days ago he blocked my number and facebook account. And said he can no longer have any contact with me, which I thought was weird because he was the one who was saying we need to be communicating. Then I found out he spent the weekend with her, he told me they are not together and not to worry.
But tonight I got a call from a friend saying his facebook relationship status changed to 'In a relationship'. he had promised that if anything like this happened I would hear it from him and not through social media.
I managed to get a hold of him and they had both decided that the right amount of time had passed to make their relationship official. Is a month after leaving really the right amount of time?

I don't really know where to go from here...he has constantly told me that he might come back, giving me hope, and now he is actually dating the girl that caused all of this drama.
And he hasn't just lost me, he has lost all of our friends too because they don't condone what he has done...it's like this girl has a hold over him that he will choose her over his life.
He threw away me, our cat, my family (who have been amazing to him), our friends, our love, our house, our lifestyle, our future children, our holiday to america next month. Our whole life is gone.

Is this girl really what would have caused him to be so rash in his decision? I've never known him to act this way, I don't even know the person he is now.

I'm trying to move on with my life but it's only been a month, I've had to move back in with my parents, I've had to cancel IVF, cancel the trip to America. But he seems to be living it up, buying new things for himself, moving on after only a month...Is this normal? or is he going through a quaterlife crisis? (he's only 25, I'm 23).

I'm so confused right now. Where do I go from here?
Update: In response to an answer I had - I didn't leave him...I would never have left him, ever. He left me. With no real reason either.
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