Extremely hard to shift, where do I start??

I was out riding on Saturday, and after coming down a large hill I noticed that shifting gears had become nearly impossible. At first I thought maybe the linkage had come loose, but after tearing the plastics off everything is till tight. If I turn the key to on, but don’t fire the engine, I can stick shift with no problems through all the gears. As soon as the engine is running, I literally have to put all my force into moving the shift stick, to the point where it feels something is going to snap. I immediately packed it up and got it into my garage before anything major broke on it. Now I’m asking to see if anyone else has had this issue, and what would be the best guess as to what is wrong?