I would check for broken engine mounts and also pull cvt cover off and check for binding clutches. It sounds like your belt is too tight causing it to keep the secondary spinning all the time. That will make it hard to shift with the engine running. If the engine is moving around it could cause the belt to tighten up. If the primary clutch is binding it could keep the belt moving. Do your ride in deep water/mud? Could have gotten some water inside the cover and caused it, Buddy.
I know the "VOICES" aren't real but they have some great ideas.
1999 Sport 400 2x4
Complete rebuild w/Wiseco piston by Todd Kerby, Comet clutch, Swamplites front & rear, 03 Predator Rear Hubs,
ITP Pro Baja Wheels (4x156 front & rear), DG Front Bumper, Fuj Rear Grab Bars and RZR Belt