Stuttering clutch
 

I'll start by saying that I am driving a 2005 Mazda 2 Antares which has done a rather incredible 211k miles, without ever having its clutch changed!! Quite recently I took a trip to London, on my way into London I was unsurprisingly met with heavy traffic, which involved a lot of clutch use, admittedly instead of knocking it out of gear I held the clutch down, after a while of being stuck in traffic (about 1 hour) the clutch started stuttering as I released the pedal and became difficult changing gear, I think it has now done some permanent damage but when left for some time to ¿"cool down"?? It would almost be normal with slight difficulty changing gear but a lot easier than when the clutch is used heavy... Any idea what the problem could be or advice would be appreciated