if you had a manual transmission, (clutch, and gear shift) for every little thing ya did. you would more likely end up stalling the tractor out, to point engine dies when running in to high of a gear and to much load on the tractor.
when dealing with a HST like transmission, it can be easy, to "over heat" the transmission, buy trying to run in to high of a gear. the engine does not die, but the hyd oil in the hyd pump and then hyd motor combo in the transmission. will force oil through seals and vary high tolerance passage ways (around pistons, and like) when this happens heat can build up quickly. ya engine does not die, but the abuse is taken out on the transmission. long term, transmission may need to be rebuilt, due to you really do not get any "tall tell signs" like an engine dieing with an HST transmission.
i never really liked a "gear / manual" transmission more so when working on hills, and then when frequent needing to change (forward / reverse) or changing gears. push in clutch change gear, release clutch, and in that split second i have just slid down hill. it may be a fraction of an inch but i have, then when i release clutch, there is sudden power to the tires, and that wanted little bit of 1/2" movement just went 2 inches movement and just ran into something.
HST and like automatic transmissions, reduce that "time lapse" between what it would take me to physically shift, vs a computer controlled or mechanical linkage controlled shifting. that time lapse is a big thing for me. ya couple HP of engine are eaten up by a HST transmission. but that couple HP for much smoother transition in Speeds and keep torque at the wheels constant is a huge plus for me.
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stop being a "ram rod" and pushing things to point of breaking, slower down. take your time. goto a lower gear.
see if your tractor has a hyd oil cooler for the transmission, and make sure it is clean of dust and grass and weeds.
see about setting up a temp gauge on hyd oil cooling line for the HST transmission. and actually get some sort of readings, you can actually judge things by. vs. just guessing.
look at proper
ballast (adding weight), either filling rear tires with a fluid, or placing a ballast box, or using some other 3pt hitch implement as rear ballast, for what ever you are doing on with front of tractor, and vice vs. when using 3pt hitch implements, see if you need to add weight to the front of tractor.
highest gear is normally not "work" gear, but a traveling gear, to get from point A to point B fast, with no real extra load on the tractor.