Yes, writing a letter is good but how about writing a letter with all the words you really want to use and say in it exactly what you think. Then, you need to make a bonfire and throw the letter in the fire and watch it burn! It is quite liberating!

You can also try other things to deal with the anger. Anger is part of healing and is not an emotion to be stuffed away. So, we need to let it out safely. You can punch a pillow, even draw a face on the pillow if that helps and give it a good thrashing. No harm will be done.

You could try using a wet flannel/cloth, wring it out a bit and then chuck it at the wall, keep chucking it until you feel better.

Exercise can be a good way to relieve the anger, put the anger to good use and go for a really long walk, run, cycle.

Find somewhere that is your safe place where there aren't many people around and then scream. Sounds odd but this can be helpful, to just let it all out.

Painting or colouring, scribbling. Any colour you want on the biggest piece of paper you can find. It does not need to make sense, just draw. Creativity can come from the anger.

Hugs.