Maybe OP had a good reason not to trust her? But you're right, without trust, there's hardly a relationship

That's part of what I don't get. If he didn't trust her to hang out with a male friend, why did he stay in an unbalanced relationship for three years? Seriously, if you don't trust someone don't date them.

Considering that the relationship has been ongoing for three years, and OP admits to being "over jealous" I wouldn't be surprised if she started spending more and more time with the guy friend because OP was being overbearing. Especially since I have seen it happen repeatedly where person a is unhealthily controlling over who person b spends time with, to the point that they push b closer to person c. It's called the triangulation effect, and I'm willing to guess it is at play here. My point here still stands- OP admitted to behavior that is unhealthy, but was surprised by their GF breaking up with them over the lack of trust. Nevermind that, if he's been having suspicions about this for as long as is implied, he should have called it off earlier. As for the GF- if OP really was pushing at her the way it sounds like he was, I can't say I blame her for jumping ship, especially since we have no idea how extreme his "over jealous" behavior was. Maybe going straight into dating the guy friend is a bit suspect, but considering it seems like OP had been insinuating to her face that she was already dating him, I could understand dating him immediately after breaking up, if only as a '**** you' to OP for the behavior. Seriously, if OP couldn't trust her that she was just friends with the guy at the time, he shouldn't have dated her. Relationships without trust are not healthy- and regardless of who is at fault here, the relationship wasn't a good one.