First, have you tried adjusting the shifter? Does double pumping the clutch pedal or bleeding the clutch fluid make any difference?
To adjust the shifter, follow the shifter cables to on top of the transmission. Loosen the 10mm nut on the plastic shifter bracket. Have a helper move the shifter while you watch it. Slightly shorten the cable travel to account for wear and cable stretch and retighten the 10mm nut. See if it makes a difference.
If this doesn't help then you can consider replacing the synchro or the gear. Here is a pic of the shifter bracket. Sometimes it breaks with age so it can be replaced for preventative maintenance.
Is it grinding? If you double clutch and rev match and it shifts fine than it's the syncrho. If you perfectly rev match than it's not the synchro. The synchro's job is to match the gear speeds for you.
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