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You haven't given us a lot of information to go on. Do you spend about 45 minutes to an hour warming up on the day of the open mic just an hour or so before you go on?
If not, then you should be.
Are you doing your workouts faithfully?
Try to keep support in mind when you first go on.
If you just start singing without proper support, you are likely to get off to a poor start and things can go downhill from there if you are singing without support.