I don't think you are being selfish or mean, but is it possible that you used a harsh tone with her? If yes, then you can say you're sorry for the way you talked to her and that you were busy. She'll get over it anyway...

You're not supposed to be constantly texting your mother. Not that you can't, but usually both sides have stuff to do and are possibly busy with their partners or other people. At least, I always think parents are supposed to step back from their grownup children's lives. From your words she even sounds a bit needy (or maybe overprotective in an intrusive way), tbh.