Heavy Clutch pedal
If you want to find the decision of this problem, you must order to diagnose clutch or you need go way more information about it.
To help you diagnose the clutch what might be any problem, it would be well for you to understand clutch construction and how clutch works. If your transmission gradually became harder to shift, the problem lies only to be causing it, the pressure plate.
So, if it`s failing that, the issue must be in the activation system itself, between the pedal and the clutch arm. There are three main types of clutch activation systems: Linkage, Cable and Hydraulic. You can learn about them here “Clutch construction“.
All possible probability, there are some issue lies either in lubrication or in some failure in the linkage. One of other problem can be having a bushing broken, that causing an inordinate amount of force to move the clutch pedal.