Don't feel too bad about hiding your anger. At least, not right now. When your spleen ruptures from internalized stress, then you can feel bad about it. Passive people act that way because they're ultimately sweet and don't want to upset anyone. While that may work for the short term, you end up looking like a real back-stabber when you, ah, stab someone in the back. Try to deal with your problems up front, and you probably won't have to renew your concealed weapons permit this year.
Your heart's a little dark, but your kindness makes up for any evil deeds (except for that stunt you pulled in elementary school — yes, that one — tsk, tsk, that was pure wickedness). But you can forgive yourself for coming off as a meanie, because if you were 100 percent sweet, you wouldn't be normal. So continue being considerate of others, and remember — when you get cut off in traffic, it's okay to give the finger every once in awhile.