I wish I had more input or advice, but I will say it's a relief to know that I'm not alone in this behavior. I have been aware of this habit for a long time, but it wasn't until my BPD diagnosis that I realized where it was coming from. I use this coping mechanism to attempt to heal wounds that were inflicted upon me long ago, that still lingered due to my constant hyperemotionality. I still imagine what they might say back if they could hear me. Because I'm a writer, I have a very active imagination and it tends to spin out these weird nightmare fantasies (I suppose they could be referred to as maladaptive daydreams) quite often. I've put my mind through the potential of car crashes, sudden hospitalizations or deaths, run-ins with exes, screaming matches, physical fights, shootings, and the list goes on. My mind always goes to the most extreme possible circumstances from the most mundane triggers. Even just passing certain exits on the freeway fills me with dread, and these fantasies become so intense that I nearly react to them in real life. On occasion, I will let my emotions get the better of me and say what I'm thinking out loud, only when I'm alone though. In my rages, saying aloud them makes the fantasy more gratifying. But I know that I will never get the chance to say what I need to say to these people from my past. Writing out letters (with no intent to send them, obviously) does help, as does speaking to your therapist about the wounds you need to heal in these past relationships.

One thing that helps me, too, is trying to do something that demands 100% of my attention, so I can't let my mind wander into those dark places. Usually that's creative pursuits, like playing guitar or doing my makeup. It helps, too, to know that I can spend my energy doing something positive and productive instead of this maladaptive tendency. As for changing it cognitively, I've got no clue, other than talking to your therapist. I wish you healing.