Oh gosh, this feeling is really frustrating when you're really trying your hardest to complete a project!
I can't really tell you exactly how many times I've felt like this, whether due to just completely losing motivation, absolutely zero feedback, or just feeling bad about my own art.
Although I can't say I've given up on any project really. This is mostly because, even though I have tons of ideas just collecting dust in various word documents, most have never made it past the 'idea' stage.
In my personal experience working as a solo dev, I find that most (if not all) of the 'losing motivation' feeling stems from the fact that I'm never satisfied with my own art and keep re-doing it until I feel that it's decent enough. But then, this leads to the problem that Mammon stated a page back, never finishing my project because I just keep re-drawing everything, even if I have everything else completely finished. NaNoRenO was actually a good motivator for me since before that, I had never managed to release anything
(sure, I only managed to release a demo, but it was the first time I actually got something out there!).
Despite my art problems though, I get that having no feedback for months or even more can be a big reason for why you lose motivation. Granted, I haven't fully completed a project at all, so I can understand why I don't get much feedback on my projects. But in the end, I really try my best to keep going despite the lack of comments because, well, I have fun working on my projects!
I feel like I'm only stating the obvious here and in kind of a cheesy way
but I think that the most important thing for you to do is to love what you're writing/working on so that regardless of the lack of feedback, interest, or whatever, you still have the desire to complete your project! I've been working on my main project, mostly in silence, for probably about 7+ years now, and I've never once lost the motivation to continue it since it's just something I love writing and working on so much!