I agree very much that at times, it can quickly seem like something you wanted to do is not really worth doing. Even when on paper, pushing through is a no-brainer, you can still second-guess yourself and really entertain the idea of giving up. It happens with me frequently when I want to start working out on a regular basis. I'm in pretty good shape to start with, but after a week or two it feels like it's consuming my life and I'll start to drift away from it. About a month later, I realize that I need to get back on the horse and thus the cycle continues.
Lately, I've really been trying to put a positive spin on my negative thoughts. For example - the other day I missed an opportunity to make $250. I was a little frustrated at first and then I remembered that all my bills for the month were paid, I have a full fridge, and I'm in good health. Given those circumstances, what reason did I have to get upset about missing a little extra cash?
It's too easy to beat ourselves up over the little things, and we tend forget what we already have or have accomplished. When you put whatever it is you're trying to achieve in the context of your life (past, present, and future), it's very easy to get excited again about the goal you're trying for. Even if you fail to meet your own expectations when you're done, you're still better off than when you started. The rewards only become apparent when you stop focusing on what you didn't do and start focusing on what were able to do. Trying to learn a language? You might not ever be conversational, but now you can get around in another country.
As far as the "never-ending path of dissatisfaction" goes, if you can break down your day into little goals you can start to see the benefits earlier. Maybe you saved yourself $20 by not buying something. Maybe you finished your work early. Or, you can look at your goals in progress in the context of the larger scope of things. You're learning how to brew beer - you've got a batch that you can crack open next week. It may not be the greatest beer you ever tried, but it's still beer and you didn't even know where to start 6 months ago, so you're that much closer to besting Sam Adams himself.
Just remember, everything is about context. If you can take a step back, look at things from a different perspective, or through a different lens, you can start to see how much better your life is today and how much better it will be tomorrow.