How do Americans feel about a foreigner making English mistakes?

English is my third language which I started learning only about 7 years ago... I think my level in English is very good now, yet when I have to talk to a stranger, and especially to a group of strangers, I get so confused and nervous, and worst of all, I lose most of my vocabulary! I start mumbling short sentances and mostly repeat myself. This problem arises when I say more than one or two sentances... I suddenly "realize" that I am speaking....and It's like I can't form a clear, organized sentance anymore, because all I think about is that others must be thinking that I am stupid, or that I must have made a stupid language mistake To make it worse, I don't have an accent, so people just ume that I am American or that I have been living here forever.. Do I have a reason to be worried? I mean I know that making a ridiculous mistake won't kill me and isn't the end of the world,but do Americans think it's stupid when a foreigner says something wrong or conjugates a verb incorrectly?