I suggested lowering the saddle to see if your butt settles back into the pocket of the saddle and you replied that you can't because it makes your knees hurt. I didn't say lower the saddle and pedal hard to make your knees hurt, I said lower the saddle to see if your butt doesn't settle into the back of the saddle where it should be. Getting comfortable on a bike is a trail and error process and sometimes you have to take one step back to move two steps forward.

Here's some other advice. Take the bike back to the shop where you bought it, tell the fitter or salesman what your problem is, have him watch you take a couple laps around the parking lot, and listen to his advice. That's his job. Getting comfortable on a comfort bike should not be difficult.