Dear crush... I don't have a crush on you anymore so stop breaking my heart. Hope she loves you better and makes you a better person.
Why do you want to write this letter to your crush?
She needs to feel pain the way I felt.
It's good to move on and not wallow in your misery over the breakup, but there's no reason to make anyone else feel your pain and suffering. Leave your crush alone and try to find other things to occupy your time so that you can heal from the emotional scars.
I still think that I will feel better if I write the letter.
You should write the letter if that is what is going to make you feel better. You should burn it when you are done though, to help you move on from the heartbreak without seeming spiteful.