If a married man and his ex mistress were friends b4 the affair and when the affair was discovered you guys stopped talking but u miss the friendship is it wrong to want to be friends again?

It's impossible to be friends anymore. The two of you betrayed his wife's trust.

If he is back with his wife, she could never trust either of you and undoubtedly doesn't want to be in the same room with you. Too bad you didn't consider the loss before you crossed that line.