Re: my bike frame is to big for me.
A 21 inch mountain bike frame is really tall -much taller than a road bike, your only option is to cut and re-weld the frame (if you don't want to put 20 inch wheels-which would fix your gearing issue, by the way).
Didn't you try to ride the bike before you motorized? I have a similar size frame and use 24 inch wheels and a low seat. I am 5'7 and this works for me.