Your mother’s words are not “nice”. They are manipulative. I’m guessing that she is terrified of living on her own. Instead of dealing with her own dependence on you, she has worked to make you feel dependent on her! She has been chipping away at your self-esteeem for years so you won’t leave her.
You are not unlovable. You have a boyfriend who loves you and wants to be with you. I imagine that it hurts him a lot that you don’t embrace his love. He may even resent it that you question his judgment in loving you.
Fighting with your mother is unlikely to get you anywhere. Her dependence on you isn’t rational. You can’t reason or fight someone out of irrational thinking. You need to shift the conversation to what is real: Her fear living by herself.
What you may be able to do, if you can keep calm, is to gently ask her what you can do to help her be less afraid of being on her own. She probably won’t like it that you finally see what’s going on. She may try to provoke another fight.
Don’t fall for it! Stay calm. Let her know you love her but you are an adult and need to get on with your life. So does she. If she continues to provoke, tell her as lovingly as possible that you do love her, but that you won’t participate in shouting matches about the lie that you are unlovable. Tell her that when she is willing to talk about next steps for both of you, without putting you down, you will be glad to do it. Then calmly leave.
The manipulative behavior has gone on for a very long time. It will take time to change it. But if you are persistent and kind, you may be able to establish a more adult to adult relationship with your mother.
I wish you well.