I know what you mean about the shame afterwards, but it's an easy fix. Next time you're in a situation where you feel obligated to act (for whatever reason), just do it. Go in with the understanding that there's a good chance you'll look awkward or embarrass yourself or whatever, but at least you did what you felt was right. Decent people will understand how hard that was for you & respect you for it. If people are going to laugh at you or look down on you for a) helping a complete stranger or b) stuttering, then fuck them. People who do that shit don't matter anyway.