flatbusher – I don’t think that’s what is going on here. I think he had concerns (possibly the whole time) but he really liked Lavender so he pushed them to the back of his head. Once it really got to the point of commitment, he had to confront those issues and he realized they really were a problem. However, he really liked her so it was hard for him to break up with her. He spoke to his Rav who gave him the chizuk he needed to break up with her. It was still a really hard thing for him to do, and it made it easier to say it came from his Rav (which was the truth in any case).
What the issues were is anybody’s guess,and there is no way to know. It could be something about him that really has nothing to do with her, but it was easier for him to say, “We are very different”. Or maybe his point was that since he has some kind of issue, he needs someone with a similar issue. Or maybe he feels there are personality or hashkafa differences that he doesn’t want to discuss.
What is important for you to remember, Lavender, is that he clearly really did like you to the point that he came very close to marrying you and would have done so if not for the fact that there were logical reasons why it was not a good idea. In fact, he liked you so much that it was very hard for him to break up and he needed his Rav’s chizuk.
I also do think that there is a very strong possibility that the issue has to do with him and not with you. Especially since if it had to do with you, you would probably have some sense of what it is which you don’t. Unlike some of the other posters, I don’t think he is unstable, but there are all sorts of “issues” a person can have without being “unstable”, and there is really no way to know what it is.
As others have said, try to be thankful that Hashem has spared you, although it is hard right now. Try to find some kind of worthwhile and/or enjoyable pursuit to “throw yourself into” so you don’t obsess over him. You have just spent the last 6 weeks on this guy; now you have time for other things. Use that time in a way that will help you to eventually be thankful that you now have time for other pursuits.
Hatzlacha Rabba!