As stated above, try the shifter on the starboard side of the tranny and see if the cable is the problem or the tranny. If the tranny is the problem and cable is free to move. Consider the below info.
The forward gear is a direct drive clutch, the pressure plate is a manual finger actuation the fingers ride up on a cone and snap into a groove to hold in gear, IF the tranny was not staying in gear the adjustment could be to tight and the fingers not able to get in the grove to hold forward gear, You may have finally forced the cone forward enough to get the fingers in the grove and if too tight the fingers don't want to slip out of the grove. This adjustment is very sensitive and one notch can be more adjustment than ideal. FYI reverse is engaged by a band and is activated by a wedge that the shifter moves to tighten the band and there is no detent to hold the tranny in reverse. IF you need to do the adjustment on the tranny be very careful the screw that holds the adjuster ring should not be over tightened as this can break the carrier for the clutch hub also only move the ring one notch at a time.