Does it run fine if you don't step on it? Can you increase and decrease speed and elevation normally?
Have you ever lubricated the deck and belt?
Well it's most likely beyond a lube issue now. After not lubricating it causes more friction and heat which damages component. Most likely your Motor has over heated too many times and needs to be replaced along with belt and deck
Try to lube it up very well and see if it makes any difference. If it does you know that's the problem
100% liquid silicone. Spray under the belt as far as you can from front to back on both sides. Then see if there is any difference