I had similar issue where I put the car in 1st and it would feel as is if it was putting an auto car in drive. I replaced the clutch cable cause of "stretching" stories that I read and it cured it for a short time. I ended up upgrading the quadrant, adding an adjustable cable and firewall adjuster. Didn't have any issues after that.
It is possible that the pressure plate could be worn out or broken and causing a very slight engagement even when the clutch pedal is on the floor. You did describe a "shaking" feeling when stopped and in gear so depending on the age/abuse on the clutch and or the adjustment results of your cable, will better determine which route you need to go first.