As
tkeithlu says "it's not rocket science....you can do it".
I don't know what brand control you have, but I myself have a
single lever Morse MV2 control. It was playing up badly so I picked up another off ebay, but in the meantime I took my old control apart and repaired it myself. There was a small spring and ball bearing which jumped out during disassembly but it didn't take a science degree to
work out how they fitted back in. It turned out that the nylon bushes on the throttle lever shaft and
gear select shaft were badly worn. I couldn't find where to buy replacement
parts so I made my own bushes using a piece of nylon strip which I shaped with a heat gun. My old control works fine now and I keep it as a spare, just in case.
You won't know what's wrong 'til you pull yours apart but it sounds like it just needs a good bit of
maintenance. There'll probably be two levers fixed to the shafts so take note, or a
photo, of the of their position before you remove them.
The cable clamp, as suggested, will solve your throttle slipping problem. Just tighten it gradually until you get the right amount of tension, too tight can damage your cable.