When I first saw the headline for this topic, I was going to say, 'Rush ahead with the next project!!' ...But, having read the post now, I see that would defeat the point, lol.
Basically, the way I try to mentally approach my projects is something like this: I find that it's directly after I've finished something up that I have a ton of creative momentum, so I try to lean hard into that, especially if it's something I'm really motivated and excited to do. Rather, it's actually more during the middle of a project that I find I need to stop and make myself take a break.
This is for two reasons mainly--one, it's the middle part of a project where the initial excitement of a new project has worn off but the end isn't close enough to provide motivation that I find I tend to get the most bogged down, starting to hate how things look. (This is the opposite of my instinct because sometimes it feels like if things aren't going well, I just want to keep forcing it to try to get things back on track.) Two, I find when I'm being motivated purely by the satisfaction of feeling finished with something I tend to rush the work and in the end don't produce something as good as if I made myself go away from it so as to come back mentally refreshed.
So I say going ahead with the next project isn't a bad thing, especially if you are feeling particularly motivated and excited about it. (Now, if you're just doing it because you're stuck in the workaholic mindset and too exhausted to change modes, that might be a good time to make yourself step back and do something relaxing.)
Anyway, some ways I like to relax are by reading and playing video games. I'll also look up ways to use tools on programs more efficiently and learn new programs on things I'd been curious about for awhile, though usually those are a procrastination tactic more than relaxing, lol. (Which, to an extent I admit I embrace useful procrastination, because there are lots of things I should do that I know I won't do otherwise.)