I guess with regard to telling your therapist, it depends how real you feel the fantasies are. If you feel these thoughts could be something, I would definitely mention it. It sounds like you acknowledge they are negative/bad thoughts and you want them to stop in a constructive way.

I read this Why Ruminating is Unhealthy and How to Stop | World of Psychology recently which led me on to trying mindfulness. Ive really not tried to address any of my bpd issues in the past but Im so sick of my life that I thought Id give it a pop. I know a lot of people say they get something from it.

If I could get on top of one facet of bpd it would be obsessive thinking. It consumes me sometimes but Im trying to think of it as a bad habit and that I need to break the cycle.

Take care.