Since then, I've set aside a few more manuscripts. Some of these unfinished manuscripts may be finished eventually. Others probably won't. I don't think this is a problem. There are plenty of manuscripts I have finished, so I don't mind that there are some I haven't—but this isn't the case for everyone.
Knowing when to give up on a manuscript can be a difficult thing. On the one hand, you don't want to waste more time on something that you know isn't going anywhere. Stubbornly sticking with it just because you've already invested so much work may be a sign that you're falling victim to the sunk cost fallacy.
On the other hand, if you never finish anything, that's a problem, too. I've heard of writers who have dozens of manuscripts they've started but no finished work to show for it.
So how do you decide whether to quit or to push forward? I think it comes down to two questions you need to ask yourself.
Questions One: Why do you want to quit?
Is it because you've discovered your manuscript is fundamentally, irreparably flawed, have learned from your mistakes, and are ready to apply your new knowledge to a superior project? Or have you realized that writing a complete novel is hard work and you'd rather play with a shiny new idea?
If it's the latter, yeah, writing a complete novel is hard. Suck it up and finish the manuscript!
Question Two: Have you done this before?
If you've fallen into the bad habit of not finishing any of the manuscripts you start, you need to figure out why. Maybe it's that you have a hard time focusing and need to force yourself to stick with the project. Find a way to motivate yourself to finish: join a critique group, set a deadline, pick a reward for yourself—whatever it takes.
Alternatively, the problem could be that you keep writing yourself into a corner. If so, you might need to scrap this manuscript, or at least a big chunk of it. To make sure this doesn't keep happening, though, seriously consider adjusting your writing process for your next project. Pantsing works for some people, but maybe you need to be more of a plotter.
If you do decide to abandon a work in progress, remember that you can always return to it later, whether it's to finish, revise, or completely rewrite the manuscript. Sometimes, you don't need to give up on a project. You just need a healthy break from it.