For me that's part of the hypomania. If you had a recent med change it does take time, but I would let your pdoc know what you are experiencing. When I'm in a mood and/or starting a new med I avoid people that are triggers for me, but sometimes I'm in the situation and it just happens. Whether you have BP or not humans can loose their temper and say things they regret after. Unfortunately part of the BP for me it can intensify those feelings and I can loose it on someone and regret it after. An apology letter is a great idea and I maybe also follow-up with a phone call.

As far as ruminating over "mistakes" I've made I can be the queen of that. It's destructive and saying in my mind "STOP" will get me out of loop and talking to a therapist also helps. Hope that helps.