This probably isn't the answer that you want, but...

Ride more. You just need to condition yourself to be a stronger rider. (At least it's free )

Having said that,

When you say you max out at 14-15 MPH, do you mean at that speed that your legs are moving so fast you have trouble keeping them on the pedals? Or, do you mean you feel like your legs can't 'push' enough to go faster?

If it's the former, and you are in the highest gear on the bike, you could change your gearing to get some more speed. At 14-15 MPH, I suspect you still have a few more gears left, so I seriously doubt that's the issue.

Can you ride significantly faster on another bike?