Buy the smurf snot.
Take the car for a short drive to get the box oil warm (not super boiling oh jesus christ my skin why does it burn hot. Thats too hot.)
Drain the gearbox fluid leave it over night dripping into a tray
Next morning plug the gearbox back up, insert the appropriate amount of smurf snot.

That should make it better and save you having to go to a mechanic straight away. Start saving for new synchros / rebuild / new box / 6 speed.
Smurf snot doesn't like to mix with other oils hence the leaving it draining overnight.

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MY93 Impreza GX 1.8L, 0hp, 0nm