Your sentence, “I had done it, you would never forgiven me”, is ungrammatical, since we cannot omit “if” or “have”. We can omit “if” if we use inversion — this is formal style going back to Old English grammar: “Had I done it, you would never have forgiven me.” In spoken English, instead of “you would never have”, we usually say, “you would’ve never” or “you never would’ve”. In informal English, you may hear the simple past used instead of the past perfect. This is wrong — I would never do that myself. You see, in theory, this should only be used in the past *real* conditional, but you may hear it in the unreal past conditional (even though it’s a mistake): “If I did it, you never would’ve forgiven me.” In colloquial American English, you may hear “would” in the if-clause: “If I would’ve done it, …”. Again, I don’t talk that way, but a lot of Americans do. Note that in some specific situations, it is OK to use “would” this way in standard English. In such cases, “would” has something close to its original meaning of “would want to”: "If you would only listen to me for once, …”.

Hey:) I feel like more context might help answer better. But the below makes sense to me... “If I had done it, you would never have forgiven me.” “If I did it, you would never forgive me.” Without the “if” I’m confused on the meaning. I also don’t typically hear or use “done it” so if the action is already complete and you don’t want to use “if” maybe it would be... “I did it; you never would have forgiven me.” “I did it; you would never forgive me.” But again, I think more context would help because I’m not certain those make sense for what’s being talked about :D