do they ever really change?

has anyone ever been in a relationship maybe it was a first serious relationship or a younger relationship where it was good and then went downhill and there was a lot of one-sided of the relationship.. I don't know have you guys ever been through crap or where one of you were unloyal and just came out of it and have actually became stronger or is that just a fantasy? Or not even cheating physically. Idk Does that really happen? can that really happen? if so how long have you guys been together? personal experiences?