Personally for me, it is the ascent that acts against my confidence. As I don't have american ascent, I feel tentative to talk in front of group of people because I sometimes feel I am not pronouncing the words right or people are not understanding the word what  I meant to say. During my first semester in my chemistry class, I pronounced chemical bond in a way it sounded chemical bone..you know  how bond is pronounced like b -a - nd sound.and my group members were like "Jesus!".That was kind of depressing during my first month in USA.When I sometimes talk on the phone, a lady picks up the phone and says "I don't understand you ".. may be because I speak fast but its the way they sound that sets me back..