A twisted mouth snaffle is not a mild bit. I would go to a simple snaffle or a French link and use that. It isn't going to change overnight. Depending on how often you ride him it could take quite a while for him to become more accepting.
I've had similar problems over the years and it's just taken time and patience for a horse to come around. I've done the "bridle on - bridle off" routine or just bridling my horse and walking him around then taking it off. BTW taking it off can be just as stressful.
What I've done as well if I know that he will back up when I go to bridle him is to have his butt against a wall. I'll also bring the bridle up to his mouth and wait. I'll let him go through his antics with me just holding it in place and not fighting him.
It takes a lot of time for a horse to overcome the fear he's built up - and a lot of patience. It's not a quick fix.