Fine line between self-doubt and self-criticism

I've been torn these past few days over a story I'm writing. To be honest, I'm not crazy about it and while I have invested a considerable amount of effort into it, I'm little more than halfway through and I just don't know if I want to continue doing it anymore.
See, I have this belief, in regards to pretty much anything in life, and books and writing are no exception. And this belief says that if your thoughts keep straying and you just can't seem to concentrate on something and keep yourself and your mind there, then maybe that something isn't for you, you know?

Oh, I'm just distracted
Oh, it's just me procrastinating

But these are just excuses, in my humble opinion. Usually, if something doesn't spark your interest (anymore), then there's something wrong there and you should perhaps consider why you're no longer interested. While sometimes the above are true and there are other things weighing on your mind or you could be interested, but you're just not putting in the time and effort, often enough, it's your subconscious telling you to move on.

So, I keep thinking "this is shit", but that's not much to go by, since I think that about a lot of the stuff I write and then come across people who seem to think otherwise. So, who the hell am I to know what's good and what isn't?

But this isn't just me being negative, this is actually not working and I think as an artist, it's important to identify that voice in your head that tells you it isn't working and to learn the difference between that and the ever-wonderful doubts that plague us all. Not all your doubts are correct, but sometimes they are and I guess what I'm wondering is, as an artist, how do you make the difference?

How do you know something's not working?

I'm honestly curious.

Then of course, there's the initial reluctance of letting go. Because even if you've learned that difference, you're still walking a slippery slope here and abandoning something that no longer works is a confirmation of sorts that nothing really works.

It's why we keep reading books we don't enjoy, keep watching the TV series that bore us stiff and stay in relationships that are no longer doing it for us. Because if we've invested so much time, then it's got to be worth something...

...hasn't it?

But as they say in yoga, let go of what no longer served and move the fuck on.

_{Well, except I added the "move the fuck on"}