I recently heard about a guy who was dating a girl and it was also going really well and they were very close to getting engaged. The problem was that the guy had some emotional issues, which he was getting help for, however his mentor/therapist/Rabbi or whatever told him that in the emotional state he was in at the time it would not be a good idea for him to get married right then. As hard as it was he had to break it off.
In that case I believe the girl knew about the issues, but if your guy was in a similar situation it’s understandable that he wouldn’t want to tell you the reason. It could hurt his future possibilities. And I’m sure it hurt him as well to break it off since he seemed to really like you. I’m really sorry you went through this and I hope you find true bashert very soon!