I = 나 WILL NEVER = (절대)하지 않겠다. FORGET = 잊어버리다. 잊다. what you did to me = 니가 나한테 한 것(짓). maybe, I WILL NEVER FORGET WHAT YOU DID TO ME, you want to say when you are very mad because of someone? then, usually korean say like that. "난 니가 한 짓을 잊지 않을거야." it feel cold and quiet anger. "난 절대 니가 한 짓을 잊지 않을거야!" it feel hot and very mad anger.

It makes sense. However, "짓", "않겠어" sounds negative (maybe revenge) in typical Korean. If you want to express a kind of gratitude, you must say like "니가 나에게 해준 일을 잊지 않을게."