I had a job that was full of ups and downs. The great days were really great, but the frustrating days were REALLY frustrating. There were many more good days than bad, thankfully.
I'm the kind of person that gets personally very involved in work, and has a hard time seeing it as "just business." When a day was particularly troublesome -- enough to make me want to quit -- I would tell myself that I could, but that I had to want to quit every day for two weeks before I did it.
You'd be surprised how many times I started the two-week clock. And on really frustrating days, it felt good to say "That's it. The clock starts now." But ultimately, within the two weeks, there would be things about the job that made me happy I hadn't left. It ended up being a very valuable tool for me because it took my frustrations out of the "immediate" category without denying that they existed, or saying something I would have regretted later.