Hi Invisiblebaron,

This is a though situation you are going through. I wish you all strength and a happy ending.

My opinion on your question if you should listen to the advice your mother gave you or stand up to her is: both?

I see your mothers point. really! I have a child myself and this is definitly something that we will go through. And I once was that young adult. With a much older boyfriend and the future ahead of me.

I see red flags in your relationship to this guy and your mother.
guy:
First the obvious: the age difference. I know it can feel like "there isn't that much difference" but there is. I think there are people out there that are capable of handling this but my experience is: there is a power imbalance. He always has your age as a kind of weapon against you.
You wouldn't belive what a good pressure it can be to be "sooo mature for my age".
You know-all my friends thought what a beautiful couple we were, but nobody seemed to notice the things he pressured me into.
So I, as your mother, would be afraid that you are heading straight forward into an abusive relationship and the first impulse is protection.

Second: "he would write me randomly and out of the blue saying how hard it was for him and how amazing I was and how lucky anybody who spends time with me is, etc."
IF you said to him that you don't want contact than this is a "HELL NOPE"! This is: "I don't respect your boundaries but I am very sweet while doing it so it's okay"

"and doesn't like that even though she said to him that I am to young for him, a few months later he found me on fb and we started talking"
That is a very big red flag for me tbh. Not because he doesn't listen to your mother, but because he didn't got into this blindly. He was well aware of your age-difference and that it is inappropriate AND THEN made steps to deepen your relationship. Someone that old looking for a young partner is no good sign...

Now, after my personal -I had to many abusive relationships to be chill on this - answer comes the big BUT regarding to your mom:

This is not her decision to make.

You know that this is problematic. But you don't seem to believe it. This is your live and your misstakes to make. You have the right to experience this **** if this is what you want to try.

So your mother can have the best intentions but not letting you make your own decisions is not healthy. She can state her opinion, she can say that she doesn't like your decision-but she cannot make them for you. Neither can I.

Wish you luck and a good journey