There are a number of things about your fit I'd need to check.

First, is the saddle the right height? Whether or not your feet can touch the ground while in the saddle is immaterial. Mine can, just, but many people's can't - when they come to a halt they step down from the saddle to put their feet on the ground. The way to determine correct saddle height is to sit is the saddle and place your stockinged or bare heel on the pedal in the six o'clock position. You should just be able to maintain contact with the pedal with your leg fully extended. Then, when you are wearing shoes and placing the ball of your foot on the pedal, your knee will be bent about 15 - 20 degrees at the bottom of the pedal stroke, which is as it should be.

If your saddle is at the right height and you still have this problem, I wonder if the problem is that this style of bike is forcing you into too upright a position for you to be comfortable. You refer to your tail bone; when I sit on a saddle my tail bone is nowhere near to being involved, the weight is borne by my sit bones, not my coccyx. Is there any scope for lowering the handlebar a little and allowing you to lean forward a little more?