What you described sounds like the clutch disc is bad or worn out. Since the engine is revving but you aren't accelerating, that is what must be the cause.
The higher the gear, the worse it will get. Is that the case? Try this. Get to an area where there is a lot of room and no one or anything to hit. When stopped, place the transmission in fifth gear and press the throttle a little. Pop the clutch pedal. If it's bad, it won't stall the engine. If it is bad, the engine will likely not stall.
If you confirm that is the issue, the transmission will need to be removed to replace the worn parts. In addition to the clutch disc, it is recommended to also replace the pressure plate, release bearing, and to check and service the flywheel if needed.
Pic 1 is the vague directions from the manual. The remaining pics are the directions for the removal and replacement of the transmission.
Let me know if this helps, if you have questions, or if I can help in any way.
See pics below.
Images (Click to enlarge)
Sunday, May 8th, 2022 AT 9:12 PM