Open mics lead to shaky nerves. The reason is because you're not playing for fans at an open mic. You're playing for other musicians who are waiting for their own 15 minutes of glory. That can add to the pressure to perform more "perfectly" than you might at your own gig with your homegrown fans.
Seth Godin would ask "Who's it for?" If playing open mics is for you to garner more fans, then perhaps another venue would be more fruitful. But if it's for you to hone your skills in the face of adversity, then seek no further.