Did you buy this mower used? If so, you may have found out why it was for sale.
When does it break the belt, when you engage the mower or just randomly, in the middle of a cut? Engaging the mower puts lots of stress on the belt and my advice on that is engage the mower at low engine speed, then add throttle. Also, get rid of the nanny-state, protect-me-from-me switches that disengage the mower.
Of course, if that's not when the belt breaks, that's not the problem. It's still good advice, though, and will also make your electric clutch last longer.
In the other case, where it breaks while running: It's hard for me to imagine how a belt can break and look like it was 'cut by a knife' unless it's running so loose that it can deviate from its normal path and contact something sharp. Make sure you have the belt as tight as the manufacturer recommends.
Also, if you're buying belts from Cub Cadet, you can probably save money by buying the belt at an auto parts store, like NAPA.
Good luck.
OEM & after market mower belts are Kevlar in neoprene
Auomotive belts are polyester in neoprene.
They are not and never will be the same, the kevlar is about 20 times stronger.
Pull the deck off. fit the belt as it would be on the mower, get on a ladder & take a photo of it.
Also take a photo of the broken ends of the belt, top view , side view and face on the fracture.
Keepers rub on the belt and "keep it in the pulley when slack".
They should not be in contact with the belt when it is tight, if they are something is wrong with the keeper or the belt run.
It is impossible for a mower engine to apply enough tensile force to snap a deck belt, they simply do not have enough power.
What is happening is the belt is getting damaged in some way then the internal fracture get to a pont where the section size is not enough and then they appear to "just snap".
Most usual is wrong routing causing too much or too tight reverse bending which causes small splits in the neoprene fill under the cotton cover.
These grow till they are right down to the reinforcing fibres then the belt snaps.
Belts are a lot more complicated than most think and in many cases the only belt that will work is the manufacturers belt because they have a customised size & layout of reinforcing.