If you are prepared to remove the gearbox from the car you can fit a new detent pring and that should fix it (if that is actually the problem).
BUT first make sure your gear stick boot is not pulling on the gear stick and that the gear stick is truly engaging the gate (it only need to be obstructed by a few mm to easily pop out, carpet in the way, insulation in the way, a tight gear stick boot may be causes).