If you started whatever you're doing for a good reason, then there's still that reason and it should propel you to keep going. Ask yourself what it was that made you start this project and why was it important to you.
Then it's legitimate to ask "is that original reason still valid?" If not, then you might want to consider deep-sixing the entire thing. But if it's still an important reason and you still feel strongly about it, then ask yourself what it is that's holding you back or making you want to quit. Has it become difficult (or more difficult than you anticipated)? If your reason is still important to you, then you can certainly overcome "difficult" - maybe you just need a different approach. Or maybe even just take a short break to regroup. But if it's still important to you, you really do not want to quit.
I'm pretty sure I wrote about wanting to quit my July race at 298 miles (with only 16 to go). People do not believe me when I say that I would have been at peace with that and was at peace if my crew had let me quit. I would not have had regrets and would not have second guessed myself. I had had 4 1/2 days to think through this even though it hadn't been something I wanted to do at that point. But remember I was doing these 4 1/2 days on fractured feet (I didn't know that - I just knew that they hurt like the dickens). So I had had plenty of time to think about it all. I had made peace with myself and was extremely happy with what I had already done so I could live with it.
But with the publicity seminar I'm going to this week, things are different. I wanted to quit but it was simply out of fear. I realized this yesterday. The reason I wanted to go was still valid. I was just afraid. That helped me regroup and reset.
Try it and see what happen.