I know exactly what you mean. There's nothing worse than sitting there an hour later wishing you'd done a bit more. All I would add though is sometimes coming out of the gym feeling completely done in doesn't always necessarily translate into having worked the intended muscle properly.

And when you say you've done it for a year I presume you mean that and something similar, as in changing your routine occasionally. A brief spell of low reps, or high reps, a different order to the exercises or changing dumbbells for barbells and back again etc etc etc etc

Anyway, apart from that you've answered your question . . . if you are still gaining, then you must be doing it right.

Cheers