Could be a lot of things, from the simple to the complex.
So best thing to do is start with the simple, which would be to check your clutch hydraulic fluid. Is it low? Then you have a leak. Is it really dark and murky, rather than honey colored? Then it might be clogging up the clutch slave cylinder, giving you inadequate clutch pressure. You could flush that old crud out and bleed the system and see how that works.
Or you could have a very worn clutch plate, with not enough "meat" left to give you a good "release" from the engine during shifts.
Or you could have worn internal parts in the transmission.
So if I were you, I'd start with clutch hydraulics. If that does no good, i'd drain the transmission and use a synthetic trans fluid like REDLINE. If that doesn't work, I think you should take it to a shop that can test the clutch to see if it's starting to wear/slip. If the clutch tests out well, then I'd guess your issues are worn internal transmission parts and that you'd need a "small parts rebuild".