You have a bad or clogged slave or clutch master cylinder
they make it so the bike doesnt jerk around and stall when shifting gears and so you can shoot out of turns
get a new clutch
if there is a clutch there, you have to.
If you notice slipping when starting off or changing gears after the clutch is completely released, if the pedal rises almost to the top before the clutch engages, if you have difficulty changing gears, or if when you press the clutch the clutch does not disengage completely.
The clutch disconnects drive from the engine to the wheels. It is required to change gears.
You may have to have your clutch adjusted,
CLutch is buggered or might just be the clutch cable
You will be unable to shift gears
No!
If the revs decrease as the speed decreases (in 3rd & uphill) then its not slipping, you don't need a new clutch, but if you are in any gear and the revs are increasing while the speed is decreasing, then the clutch is slipping.
Sounds more like a clutch slave cylinder.
You may need to adjust your clutch or replace your clutch.