Transform the letter, and all the thoughts and pain it holds, by using one or more of the four elements (fire, earth, air and water) to render it neutral and harmless.
In other words: Burn it. Bury it. Tie it to a helium balloon and release it, or shred it and let the wind carry it away. Drop it in a river, or rip it up and flush it down the toilet.
Notice any changes in your body sensations and emotions as you render the letter . . .
You can create a whole ceremony around this if you wish. Follow whatever you are guided to do in the moment, in service to letting go.
Give it the attention it needs to be truly healing for you.