Grace, you know the letter is a bad idea. He isn't going to receive it in the manner you intend it. Write the letter for yourself and then put it away or burn it. It will just make him mad or be perceived as pursuit and let him see you sitting there nicely on the shelf.
You have to figure out what works for your thoughts. Some people snap a rubber band on their wrist, others picture a stop sign over the head.
What I do is a bit radical and might not work for everyone (or even anyone). I just walk through the thoughts and put them in what feels like a proper place for me. Then I think of having that mean, resentful man back in my house and I shudder.
Trial and error. At one point I thought about making myself do 10 push ups every time I thought about him (like a Pavlov dog training kind of thing). I never implemented it, but I thought about it. For a while I thought of him as dead. That was somewhat helpful. Mix it up. Stay busy. Do meditation, listen to podcasts, walk, cry in the shower. Just do. Doing something usually feels better than doing nothing.
