When Im near my crush which is everyday we sit together at lunch we have and two classes together plus mostly the same friends He said that I can be mean and curseful but you can be kind and sweet My problem is every time I am near him I automatically Start to say bad things How do I stop that it's only when I am around him.I am 12 girl

It's simple. You just stop. When you're near him, watch what you say, talk the way you do with everyone else.

Are you saying you cannot control your mouth when he is around? You really should try harder. If you cannot say something decent then shut up. Personally I think you are trying to hard to get his attention.

You are 12. You have a lot to learn about relationships of all kinds outside of family.
That's why boys and girls should learn about each other in groups, not alone.
You are mean to him because you are so thrilled to be next to him. You have to push him away, hurt him, so you won't get too close.
As the others said, just stop!
You can even tell him that you are mean because you like him so much, and what that puzzle means. He can learn from that too.