I’ve seen this in action and it’s heartbreaking.
I’ve found that when someone believes they are not good enough or are unloveable, the expectation is there that they will eventually be hurt. There’s no way they can fulfill the expectations of a relationship or a partner, and sometimes they try to get out sooner rather than later.
My partner tried to call off our relationship a couple of times near the beginning because he was sure he wasn’t enough.
Seven years later, those fears are still there, but he’s working on it, and we’re happy.