If you don’t delete it, she could log onto the servers and follow it. I would say to make a different account, and announce publicly that it it you with that new account. Make sure that you do not post any images or comments on your old account that can track you to your new one. And to prove it’s you, and not some impersonator, you could post a random string of numbers with your new account and then your old one (inside a post talking about how you’re leaving) and if the numbers match it shows that the alt is really you without having a way to track it to your old one.
Or, you could just, like, tell us who your alt is and then delete your account, so the post is seen by us but not your mother and can’t be seen anymore bc it’s deleted.