Either your shop sold you, or you chose, the wrong size. The 21" DS is too big for someone 5' 11.5.
If I were you, I'd cut my losses and change to a new bike, one that is the correct size so that it can be properly fitted to 'you' so that you
as you are are comfortable. You might find this link interesting:
http://www.peterwhitecycles.com/fitting.htm