Some people have more difficulty with transitions than others. You and your parents delayed the normal separation process for many years because you lived at home until your mid-twenties. Unless you were willing to live with her until she died, this time would inevitably come at some point. That point of “leaving” is now.
It sounds like your mother sees you as her friend, not her daughter, and that she is struggling mightily with being “left.” She has forgotten that it is a parent’s job to launch their children into the world, not to hold them back. She needs to be leaning on friends, not on you.
The kindest thing you can do is to be kind to her; to reassure her that you love her; and to be understanding about her panic. You have nothing to feel guilty about. You don’t need to be upset because she is. When she texts or calls, just listen, reassure her that you love her too and then change the subject if you can to things that are going well in your life. Don’t argue, debate, rebut, or fall into making apologies for being in school. Just tell her that you know it is hard for her and that you appreciate whatever support she can offer during this difficult time of transition. Ask for advice if you need it. But don’t then argue with it. Just listen respectfully and tell her you’ll think about whatever it is she seems to want you to think about.
Do remember that thousands upon thousands of families are going through something similar at the beginning of the academic year when kids move away. Your mom is just a more extreme example. Almost everyone makes it through and are glad to see each other once the holidays come around.
I wish you well.