I feel so bad..

I feel so bad.. Today on my way out of school, I went to a store to grab something to eat, I noticed that 2 people were talking about a subject that I know alot about and have experience in. All that was going in my head was "Should I join the conversation? No, maybe ill stutter badly. But I should challenge myself..etc" I ended up just staying for 2minutes listening to them talking about that subject and me burning inside that I couldnt get myself to speak. I just paid for the red bull and left. My whole day was ruined. It was fucking hell..