“Hydraulic transmission fluid”???? The transmission on that tractor is designed to sue 80W90 gear oil, not hydraulic fluid. The hydraulic system is separate for the transmission and it uses AW32 hydraulic fluid.
It does sound like your clutch may be slipping. It also may need adjusting. Do you have all the manuals for the tractor and engine?
You didn't inadvertently lock the brakes did you? Just checking, it happens.
Does you tractor have the shuttle shift option?