It might seem like just after you've achieved something is the best time to relax, but in a sense I think it's actually the most dangerous. Sure, there's a nice space for a break because your workload has suddenly just decreased, and if you've been pushing hard to get the thing done, that pressure is now gone. Maybe you need a bit of compensatory downtime to make up for the overwork. Those are all compelling reasons to take a break, but nonetheless doing so right after you finish something may be exactly the wrong time.
The first part of this issue is that if you feel an overwhelming need to relax because you've been working in a way that's been harmful to you, then that relaxation is masking an important question: why can't you keep going? Sure, for some things it's unavoidable, but I suspect fewer than we think. Sometimes you have to sprint, but mostly life is a marathon. If you have to go recover for a day every time you do something, maybe you're doing it wrong. If at the end of a project you wish you never had to do anything ever again, it's probably a better time to embrace that negativity and use it to reflect on how you can avoid it in future.
Another problem is the way the end-of-project break builds associations. There are two different good feelings in play: the good feeling of having accomplished something, and the good feeling of not doing anything. If you put those together too often, it becomes hard to disassociate them. Every time you do something is like a little signal saying "great, pack it up, we can stop now". Worse still, thanks to the perversity of two-way association, it might start to feel like not doing something is an accomplishment itself. That's a lot of impetus to stop even if you don't need to.
The last thing is that excessive relaxation is very disruptive to existing habits and patterns. I'm not talking about taking it easy for a few days, more like massive go-hide-in-a-cabin-somewhere breaks that sound suspiciously similar to burnout. You have to overcome a certain degree of inertia when you start doing something new, and it takes a particular and unique kind of effort to do that. It's much easier to maintain an existing effort. Even in a world where it was just as efficient to work twice as hard on odd weeks and take even weeks off, I still think you'd be worse off because of all the momentum you lose.
I think all of these factors contribute together to make relaxing just after you've finished dangerous. It's often a response to bad working habits, encourages you to stop when you don't need to, and kills your momentum. So what's the alternative? Never take breaks? Obviously that isn't going to work. I would suggest instead that breaks should happen in small doses while you're working on something, rather than as a big chunk at the end. Even if you're taking a holiday, I would suggest deliberately leaving things in a still-undone state before you go.
While this sounds counterintuitive and perhaps goes against some fundamental desire to tie things up neatly, I think it is much more compelling as a model. You're not taking a break because you're done forever, you're taking a break because you want to come back to what you're working on refreshed. Unless you're totally done with this project and anything coming after it, it's better not to end it too cleanly. Leave a little bit left to do, or if you've finished this part, start on the next one. That way it'll still be in the back of your head somewhere, and be much easier to get back to when you're ready.