how do i power a 12 v motor forward and reverse, using pulse wave modulation on reverse only?
An H-bridge driver is needed to replace the relays - they burn out ;-) when used this way. (@mdlrodgers above)

You can look for a RC car 'esc' (electronic speed control) from tower hobbies then drive with a single PWM that is switched 37.5% or 100% (50% is zero).  

Something like this can also work https://www.olimex.com/Products/RobotParts/MotorDrivers/BB-VNH3SP30/open-source-hardware. 

You will need to configure the signals to get it working - you could even use a 555 timer and switched resistors to get the pwm signals.

There are many other similar products from Sparkfun, Ladyada ebay , DX etc

I would suggest (and have the junk box to prove it) that you start with a motor control module capable of handling double (or more) your intended/measured current. Reason is the back EMF you get when changing speed changes the battery voltage perceived by the controller.