I'm mad at my mom for not letting me spend the weekend with my gf
I'm sure she had a good reason to not let you spend the weekend with her. A mother tend to know best.
My mother's being so selfish about this whole thing
I can understand why you are so upset about this. Parents have reasons for their decisions and you should respect them. If you are mad about the decision maybe you can ask your mom for a reason why she doesn't approve.
It's either that or I smear my mother.
Trust me, it wouldn't hurt to hear her out. At least you will get a better understanding of why she did what she did. Sometimes we are too immature to see the clearer picture and parents will make the better decision for us.