Well you're probably not going to like my answer but here goes anyway. 13 is too young to be in a relationship...... If you don't want to spend time with her anymore then just tell her the truth. If she already said she will hate you forever if you break up with her then so be it. It will be her loss....
I still like my girlfriend but I want to break up with her but I'm scared she's gonna hate me forever. She said that herself..Btw I'm thirteen and we just kissed today. I really don't know what to do. I'm helpless. Please help urgently. What should I do?
You say that you still like her. If this is true, why do you want to break up with her? And why would you kiss her if you're just about to break up with her? Also, 13 is way too young to be dating anyway. To me, it sounds like you're both being quite immature.
Then breakup with her if you must. Maybe you are feeling overwhelmed with all the relationship stuff because of your age.
13 is a bit too young to have a boyfriend/girlfriend, just tell her the truth I'm sure she'll understand :)