What should I do to prove that I forgave someone?

Nothing, because I know when you forgive someone, my child.

I don't think you need proof. As long as you really forgive yourself, why do you need proof? this is not your fault. If he still thinks you have not forgiven him, then he is not your true friend!

I need to go to sleep for a while. The way you show you forgive is you no longer bring it up, you no longer care about it. You accept, that things happen, whatever those things may have been, nobody is perfect, and you forgive him more for yourself than anyone else, because you have to let those feelings go, or they just bother you.

Nothing. You do not need to prove it. If the other person doesn't believe you, then there is something wrong with them, not you!