I'm responding, not because I have an answer for you, but because your post could have been written by me. Same scenario exactly, except I'm at that year and a half mark when he left the first time & is now wanting to get back together. He has no explanation whatsoever for the split, except to say that he "doesn't know who that person was," and knows he made a colossal mistake. My family & friends are very upset with me for opening myself back up to him.
I have analyzed it ad nauseum & have not come up with any definitive "aha" explanation.
So I suggested counseling. We have gone for one session, during which I was able to open up, and he was not. I was very bothered by this, and he didn't see how out of balance it was. So I suggested that he have a solo session, hoping that the therapist can pry him open.
I'm beginning to wonder if our approaches to emotional attachment are so diametrically opposed, that I'll always be waiting for the next time he leaves. Trust and loyalty are so important to me; doubt and fear would be a killer.