I can certainly verify this from my own experience. When I do something, not because I want to, but because I "have" to, I get fed up. I do just enough to assuage the guilt and then abandon it to go do something I actually want to do. Often, I'll just *distract* myself from feeling guilty by becoming preoccupied with something else, and then, whoops, look where the time went. I think that this is what most of us are doing when we claim we were too busy to do something. We didn't want to do it, so we let other things interfere with our ability to do it. Sounds malicious, doesn't it? It's not. What's malicious is that we don't feel free to admit that we just don't want to do it.
There's more than one kind of not wanting to do something, of course. Sometimes it's because we know it's going to be work. Ayn Rand called this the "white tennis shoes" when it comes to writing. This is where willpower can actually be useful, because if you're like me, once you get started doing work you enjoy, you'll go and go and go and not notice--because it's fun. But it takes a constant exertion of willpower to do something you *don't* enjoy on *any* level, like, for me, doing a regular diet and exercise regimen. I *don't* like being hungry and nothing I can do will ever change that. The only diet I could ever successfully do is one where I'm *not* hungry and I don't have to spend large quantities of time thinking about it. Exercise is the same way. Oh, I enjoy *specific* activities, but after three or four days of a regimen I'm bored and it starts taking constant willpower for me to keep doing it. I'd need a "regimen" that was completely different every day--but that didn't require me to sit around trying to think up how to make it different, because I'd get bored with that, too.
As a side note, I know those things aren't impossible. I've been looking at the idea behind the "paleo" diet and I think I could do it if I had more control over what I was eating at the moment. Doing a sport or martial art would be sufficiently varied for exercise, too. I've done volleyball before and I didn't want to miss a practice no matter how much work it was. We only practiced twice a week, though, so it wasn't enough.
Anyway, the real way to avoid this problem is to avoid the trap of duty-based ethics altogether. "I really should" won't turn you into a moral person. Only rejecting the evil as truly *undesirable* leads to full morality. There's a great quote in Atlas Shrugged that conveys this idea completely (on page 720 in the Centennial Edition):
[A]nd they looked as if, should they encounter malevolence, they would reject it contemptously, not as dangerous, but as stupid, they would not accept it in bruised resignation as the law of existence.
Full morality *is* possible, there is no "moral thermostat" that prevents people from becoming too moral. You just have to know that evil is not desirable and that you do not become good by grudgingly rejecting it. Nor is good some duty you owe someone above, beyond, and against what you want--not some unpleasant chore to be performed and dispensed with as soon as possible. Once you fully understand the consequences of evil, you'll never feel tempted to engage in it, and you'll find it bizarre that anyone ever could be.