And he is OK with that, does not think that is strange, or you for that matter? You are not a teenager.
You have been seeing this man for four years, he does not think it strange that at your age you are so afraid of your mother?
If you had a child, would you be able to defend that child against your mother?
You need to put on your big girl pants, act like a grown woman and tell your mother you have a man.

