Sounds like a worn synchro to me.

Synchro rings are usually made of brass, and act as friction material during the gear-change process. That is, when you try to push the shifter into a gear and it feels like it hesitates and then drops in, that hesitation is the synchro doing its job. It's using friction to rev-match the clutch's friction disk and the transmission input shaft to the speed of the counter-shaft, axles, front wheels (which are all in constant mesh) and thus speed of the car. If you push the shifter too hard, you will overpower the synchro's push-back and start grinding the teeth between the shift collar (the transmission part you're actually moving when you push the shifter forward and back) and the corresponding teeth on the gear you're trying to engage. The teeth each have little V peaks on them that let the collar and gear easily align and slide together once the synchro finishes its rev-matching thing. Grinding the gear wears the points off the V's. A worn gear/collar set can sometimes land worn-point to worn-point such that the teeth won't then align and drop into gear, or will require excessive force even though the gear isn't grinding.
Synchros are cheap but labor intensive (remove transmission, open, disassemble gear stacks, reverse) to replace. Replacing gears is the same job, but gear sets are more expensive, harder to find, and just plain an additional cost on top of doing the synchros (might as well do all of them while you're in there).

TLDR: Time for a good-condition used transmission, an already-rebuilt transmission, or DIY transmission rebuild (if you're automotive-skilled, have space to lay everything out, and can carefully follow instructions).

 

do you know how to double clutch?