I have seen this "trainer's disease" of doing things "one more time," one too many times. The worst part of this syndrome is it's easier to detect in someone else than yourself.
I think it's human goal orientation that pushes us to chase that extra bit of improvement that seems just around the corner, when the very action of pushing sets that goal even further away.
So, I promise to myself and to my little dog that I will put a timer in my training bag. Set a maximum time for each training session, say 5 minutes. I will keep only a limited number of treats on me for this time period, and when they are done, we are done.
Finally, I promise to be more observant of my dog. To heed when he's telling me he's had enough, and try harder not to even make it to that point.
Trainer's disease is curable. And the cure is keeping the dog's needs first above the trainer's.