2007 mercury optimax 90 hp will not go in reverse

I bought my 2007 mercury optimax 90 new in 2007. Warranty is expired and it recently got very difficult to shift to reverse. I took apart the control box and disconnected the shift cable from the motor. I moved the control forward, neutral and reverse, with no problem. It seems the shift mechanism on the motor is not working effectively. Can anyone help with suggestions? Seabordo