I had an affair with a tutor when I was in my teens and need some advice?

This wasn't your fault - yes you were 18 but not only was he a more mature adult, he was still in a job that requires duty of care. It sounds like you were going through a very rough time but you shouldn't be beating yourself up about it. You sound like a wonderful person and i'm sure that your kids are lucky to have such a caring mother. If you are thinking alot about this episode of your life, prehaps in order to fully get it off your chest you should go to a listening ear, maybe some counselling just so you properly leave it in the past. I wish you every luck, enjoy your life- you're a better and stronger person than him!