Did you know that I am mad at my boyfriend?
Why are you mad at your boyfriend?
He called me a liar on the phone.
I'm sorry that happened. It's okay and normal to be upset with him and you should definitely try talking it out with him.
He's not important to me. I'm going to cheat on him with my ex.
You're completely allowed to be upset with your boyfriend, but if you want to cheat on him and don't believe that he is important then you should break up with him. It isn't worth having a relationship that you're not feeling serious about.