You mention BPD, so just wondering have you tried dialectical behavioral therapy (DBT)? That's what's supposed to help most with BPD and emotional regulation. I haven't tried it myself, but have heard good things about it.

About your boss, maybe it would help you to write that letter but not send it. I've done that before to express my feelings about people who have hurt me, and it helped me to write it all down even though they never saw it.