Yet, how does my BS know that I'm changed? I come to him with uncomfortable things - I try not to conflict avoid. In fact, I barge right into conflict..
Most likely a good start.
First off, you never know. That parts sucks and you just need to get beyond it. I found that I needed the trust in me again more so than the trust in her. What I mean is I know I am good either way, with or without her, and that has given me the desire to try again with her.
We have identified (she in IC mainly and continues to) certain things about her that allowed her to make those choices. Conflict avoidance, holding on to resentment, internal dialog and to be honest reality testing for her. She would just make shit up about me and what happened and the apply some hosed up chop logic to how I was a bad guy instead of just addressing the issues with me. We identified those and now she talks to me about those things. That gives me an indication that she is processing in reality instead of in her childish fairy land.
A second reason I am confident, is we just pull each other aside now when one of us is not 'right' with something and we talk it out. We both do this, but she does more so than I. Again, another little confidence builder and thing she was not doing before.
Another thing we do is tell each other when we feel disconnected. I can tell you this - for the rest of our M if I don't here her say that at least once a month (for a long time more often than that) I will just say to her I think something is up. We have 3 little kids, she is earning her PHD and I have a busy career and social life. We need to reconnect often and she knows it.
Those are just some of the things we do, most of it is on her. There is other stuff but if I needed to point to a few that a indicators of her continuing to work on the core issues we have identified in her that allowed her 'how' and 'why' those would be them.