I don’t know why seeing you with a good man isn’t enough for her. As you’ve already discovered, age isn’t as important as character. You’re both in your twenties. It’s not like you are dating someone old enough to be your father or grandfather!
A good friendship is the best way to start a relationship. You and your boyfriend are making sound decisions about finishing school. You’ve taken two years to get to know each other and to be sure of each other. All that looks reasonable. I wish your mother could share in your happiness at finding each other.
Since she can’t, all you can do is be respectful and kind with her and go on with your life. There’s no point in responding to her unreasonableness with some of your own. Anger will only prove to her that you are too young to make a good decision. The best thing to do is love her and yet go forward. Hopefully, she will come around if she sees that you’re happy.
I wish you well.