I'm mad at my mum
Why are you mad at your mother?
Because she calls other people and tell them how much of a loser I am
If your mother is verbally abusing you or taking about you, then it's okay to be upset. You should bring up why she shouldn't call you a loser to her.
I'm not going to do that, she would never understand
You are within your right to be mad at your mother.   You need to confront her and let her know that you will not tolerate it.