Hi
Looks like that belt has been the victim of a seized pulley or or obstructed blade, it is worn away not sliced. The wear is on the inside indicating a vee pulley issue and not an idler pulley or guide issue.
I've found the easiest way to get belt tension improved is to order a belt one or two inches shorter than specified eg A 81 or A 82 instead of an A84 as an example.
Ensure the guides around the crank pulley are close, almost touching the pulley to prevent the belt dropping down and been worn on the shoulder of the pulley.
They can be millimetres away because the slack occurs on the front of the pulley furthest from the deck to cease drive transmission to the blades.
Hope my waffle all makes sense!
