Tired of having to argue with people

I'm not very good at defending myself. It usually leads to an argument. I'm tired of people saying that I do things that I very well know I don't do and they don't even believe me. My boss treats me like I'm stupid. I requested 5 days pto for vacation and she has to approve it but I doubt she is going to because she doesn't like me. She argues with me and I do have respect for her but when someone treats me like crap I try to give it right back to them. I know I should be the bigger person and just drop it but I've done that all my life and all it has done for me is let people walk all over me. I need to start defending myself.