Is it the gearbox or me ?

[unfazed]

Best thing to check the simple thing first and if the problem still exists after all this then we can investigate further.
1. Is the clutch lever adjusted properly there should be 5 to 7mm of play where the lever meets the lever holder. If not adjust the play using the adjuster on the lever bracket
2. Is the punch mark on the gear change shaft lined up with slot in the gear change bracket on the shaft. If not remove it and line it up.
3. Adjust the lever to suit your riding position
Put the bike on the centre stand then sit on the bike. Now with you in your normal relaxed riding position, check the position of you left foot in relation to the round edge at the end of the gear lever. It should be about a half inch from the bottom of the sole of the your boot. If not adjust the linkage
The linkage can be adjusted by turning the bar between the shaft and the gear lever.
Loosen the 10mm nuts (rear one is left hand thread normal tightening direction is loosening) and turn the bar to adjust the lever up or down. When you think it is the correct position for you tighten the nuts and take it for a spin.
This is a trial and error job and may take a few trys to get it right.
Let us know the result of the above.