From what I recall, these are very low end shifters and, like many low end parts, really aren't designed for long term maintenance. I think I've busted a couple myself.
The good news is that grip shifters are cheap and readily available. Sure, you can buy a new one - or if there is a co-op local to you they probably have a bin full of these that can be had for a couple bucks each. Even more thrifty, maybe you can get lucky and find a free bike somewhere and salvage the shifter from it.
Good luck.