Boredom
Rotorvation,
Have a look around you and wonder what it would be like to do "this" guy's job, or "that" guy's job. After 27 years of flying I have still not found a job I'd rather be doing.
Now, I was terminally bored at one point, teaching flying instructors. It was a great job and very worthwhile, however, it was not exciting. After that 3 year tour I left instruction altogether and got back to operational flying and that did the trick for me.
Some of the posts above offer quite clever solutions to boredom. I guess that if you are in an instructional position and not enjoying it then you have to look at either another job or a variation on your current job. Speak to your boss. He may well understand.
Just remember, we are very fortunate people and we are in the privileged position of being able to do something quite unique. If your current job doesn't do the trick for you anymore, have a look elsewhere in the industry.
Good luck