Sometimes I’ll get really angry, but instead of doing anything, I go to my room and think about how I could’ve said what I really felt. Ex. You think your life is hard? Do you know how much I go through that I NEVER even tell you about? I don’t even complain?
If this isn’t normal, does anybody have an idea what it could be?
I can't speak for normal but I know I do that all the time. One of the most common (false) myths about psychology is that talking to yourself means you are insane. So long as you are aware whether you are speaking to a real person or a mental projection you are fine.
I do this all the time. Levi-UR is right, it's not what I would call abnormal I'd say. I do this a lot when I'm angry or upset and have nobody to vent to. My head just argues with itself in made up scenarios so I have somewhere for my frustrations to go.
Normal is a stupid word, honestly. It is perfectly fine though to argue with yourself in your head. I do it all the time as well. It helps you sort out your thoughts, and automatically divides up the pros and cons, or the two sides of your argument. It helps you pull a conclusion. Also, everyone thinks back on arguments and wonders what a better / smarter response would have been. You essentially help yourself process it and get over it.