You don't seem stupid, first off. Your lack of response to things that make you angry could be due to many things. Generally most people tend to withhold an outburst due to fear of the consequences if they did speak out. Sometimes fear of losing control, the other person's reaction, or fear of the strength of you anger can cause you to hold back.
At times, I find it helpful to do something that either lets me blow off steam (exercise, cleaning, or even going out to the car and screaming loud. (Yes, windows rolled up) or journaling. Otherwise, I tend to slip into a downward spiral downward quickly and my inner negative tapes go on blast and repeat further dragging down my mood.
Also, spending time with a pet seems to help as well. I get unconditional love from my dog and cat (usually. Lol) and I feel like they know when I'm upset and want to comfort me.
I can understand feeling stupid when you can't get the words out like you want. Give yourself a break. I encourage you to work with a therapist on using 'when you ________, I feel ____ because _____' statements. You can practice with him/her saying the confronting statement as well as reviewing your fears about the consequences of opening up. Then comes the big test - actually using the skills you're building. Good luck.