I feel mad at my girlfriend.
Is there a good reason you're mad at your gf?
Yes, she borrowed my game without asking, I was careless enough to lend it to her.
Just let her know that you do not like that. You should not be overly mad about this.
I understand what you're saying but if you knew the kind of person she is, you would understand why I am so upset.
Tell her that next time. Maybe she did not know the boundaries of the relationship  yet.