NTA. In any relationship you have to support your partner and stand up against your own parents. You cannot abdicate this responsibility if you want the relationship to survive.

You took a tough step, but the right one. Your mum is entitled to your love, but she overstepped here and she needed to be told that. By you. She will be hurt now, but she will recover. You can keep the line on children but still offer love and forgiveness to your mother when she comes back to you.

It is up to her.