Well I feel that I can give some advice here for you as I was the person who did the breaking up and did it in a similiar situation. When I did it, I was in a point in my life where stress was overwhelming me, family was pressuring me to break up, and plain and simple, I was just in a bad mood for quite some time. I think you are doing the right thing by working trough this time in your life by keeping yourself busy and by talking about your issues. I broke up with my ex a while back, beginning of this year, and I now realize what I needed was some space from her, my family not badgering me any more and to get over the stressful time in my life to really understand what I wanted. Perhaps this is what he is going through. Usually when a guy breaks up his girlfriend "on a whim", it is because other factors are affecting him right now. He just needs time to be who he is to figure things out for himself. If he is a genuine guy, and yes I know right now you probably don't think so, but if you look beyond that, he just needs to be himself. I personally think that perhaps he was a little shakin' up with the idea that he has found the one that he'll marry. Many of my friends broke up with their girlfriends prior to takin' that big leap. I probably won't be the first to admit it, but guys do get scared and we do run. We are not the brightest people in the world when it comes to certain things and we do react unrationally. All I can say, and I will say it again, give him some time to get his ****** together, and perhaps it wouldn't be such a bad idea to keep yourself busy like you have been and in time, when both of you are feeling better you'll or he'll do one of two things a) let the relationship go or b) fight for what you believe in. But remember that the decision is yours and the ONLY person that has to live with it is you.
I don't know if this will help you, but I have been posting my thoughts and feelings on this board until "I miss her...". This might give you a guys point of view. Listen I don't know if anything of what I said helps or makes you understand what we guys think, but I think you might already know what you should be doing, just go out their and do it. I truely do hope, with everything I have, that you are feeling better, we are all here for you and will be here for you every step of the way. It has helped Kerry and I am sure it will help you too.