That sounds to me as if the belt was slipping on the engine pulley, at the front of the tractor, when you went forward, and was not slipping when you went backward. That would probably mean that your automatic tensioner is not working properly. (It is possible, though less likely, that the belt is worn out, and is touching the bottom of one of the pulleys, so it does not jam in the V to deliver full torque to the pulley.) Most likely you just need to lever the tensioning pulley (under the seat) forward a bit until the ratchet clicks into the next notch. The symptoms are wrong for a worn out forward clutch.

The HD8 has two V belts driven by the engine, like most ride-on mowers. It is moderately unusual in having a horizontal crankshaft engine, but like most such mowers it has two pulleys on that crankshaft, one to drive the cutter and one to drive the back wheels. If you have not read this thread about the HD8, I think you may find it interesting:
https://www.outdoorking-forum.com.au/forum/ubbthreads.php?ubb=showflat&Number=40749&page=1

To post pictures you use the fifth coloured icon from the left, at the top of the posting window. The icon looks like a spray can with a blue arrow pointing upward. You "Browse" to your picture on your hard drive, then press the button to add it to the thread. When you have attached all your pictures that way, you press the "All Done" button. You can add up to ten pictures to one post.

I am sure I am not the only one who is looking forward to seeing your pictures - the HD8 is old enough to be a very interesting machine.