I was in this situation once with a guy I fell really hard for. He and his BF split up only a few months before he and I met, so that should have been a yellow flag to start with. But things didn't seem quite right, and it turned out that while he was seeing me, he was also desperately seeking to patch things up with his ex. He broke things off with me when there was a glimmer of hope that his ex and he might patch things back up; when that didn't work out, he suddenly wanted me back. Then, one day, he completely disappeared, without any word about what happened. The logical assumption would be that he got back with his ex, but I'm not so sure about that.
It was a hard lesson. I made the mistake of getting too attached to a guy who clearly was still caught up with his ex, and I was just there to fill the gaps. Looking back, there were times I felt really stupid and a bit ashamed that I allowed myself to be used in that way. I've gotten past it now, but it took a while.