Lots to check. In the engine bay you'll see the gear change linkage, you will need to remove the battery and battery tray (and washer bottle if it's there) to get at it. There are three ball joints sitting in plastic sockets on the main for/aft rods. If any of these is failing it'll lead to issues with gear selection. Next there's a vertical rod at the front of the gearbox. Check the bushing at the base hasn't failed. At the top of this rod is another small ball joint, check for play here too. There is a cross link near the pivot on the subframe behind the gearbox, check that the ball joint at either end hasn't got excessive play. You're unlikely to have significant issues with play at the gearlever end but pull the cover off and check there's nothing loose there. Finally, there are two gearbox mounts, one at the front, the other at the back. If the rear one has failed it may have allowed the whole gearbox to tilt down which may be causing your issues.
Guy