Are both functions very stiff? To check, press and hold the shift override button and move the control handle . If it is still as stiff as before, the shift may be fine. There is a tension adjuster screws but note that it affect the entire motion of the handle. If the handle moves easily between neutral and the point where the shift engages but then gets hard to move, you may have a problem with the shifter linkage in the motor. To check that, you need to disconnect the clevis fittings at the motor ( those connect the cable end to the motor linkage) If the handle at the helm is still very tight, you probably have a problem with the control) If the handle moves easily, the tension adjustment it not the problem.
Other less common cause may be a too tight bend in the cables usually the result of using the wrong length cables.