Why won't my car shift into gear?

The other day I had been driving for about two hours and parked my car in gear. I then started driving again and went to shift into second gear but the car wouldn't go into second, so I tried first again and it would not shift into first or any other gear. After a while it was fine and I had no further trouble until I went to start my car next, when I started the car it would jump forward and when it finally started properly I couldn't get into any other gear again but it was eventually fine again. Then the third time I started the car it would not go into gear at all and has been difficult if not impossible to get into gear since then. When I have my foot fully compressed on the clutch the car shakes like it's about to stall until I pull my foot up a little. I have just had about $500 of work done on the car and am feeling that I may need a new clutch which will be about $1100. What do you think it could be?