Sounds like a clutch hydraulic problem.
You mention fluids are new, does that include a brake fluid flush? What kind of fluid were you using? Brake fluid is important on the track especially if you were braking hard, it could be possible that your brake fluid started to boil. (Your clutch shares the fluid with your brakes). That would cause the system to lose pressure and make your brakes and clutch feel spongy, possibly not enough pressure to fully engage the slave. Hence it being hard to put into gear.
How was your brakes feeling during the times you had this clutch problem?
Other issues that could cause hard gear changes at high rpm, is a beat/old/worn transmission with worn out syncros. The transmission fluid plays a role as well, because it sounds like this only happen when things heat up, so it could be you are low on fluid or it can't hold to the abuse.