I am little late in this thread, but I also totally relate. However, I do not quit when I start something, even if for a matter of principle. What Ken Gilliland said is really how I see it. I made a commitment when I became a vendor, and that is to always complete what I start. This becomes easier with experience, because now I already know how hard something will be based on the tools capabilities - before I start. I know what works, and what doesn't, and I take that into account before I start, so no surprises. I create contents for both Poser and DS, and each have their pros and cons. In other words, it becomes easier to manage content creation when we get more experience, because we reduce the chance of falling into something unexpected we won't know how to handle.
Experience also helps with knowing our own limitations. I can create anything, but I am not good in everything. I used to only choose the things I was good with, but over time I started diversifying as means to expand my product line. Your first attempts won't be the best, but it's a beginning. Practice makes it better. I was scared to death of body morphs, but now I have over a dozen of them in my store. It's that thing about facing our fears to defeat them. I am scared of character face morphs, but I have been playing with it more, with hopes to develop skill and get better. At this stage, I am not so scared anymore, but it's still hard. It will get better in time.
If I get sick of the product, take a break from it and go do something else. I do promos to escape the challenges and also test the product. I don't "love" doing promos, but it's a necessary evil, so I use it mixed with testing. Test renders can become promos with a little help. There were products I had already finished and beta-tested, but couldn't get passed the promos - because I loath doing them. To me, making promos is the worst part, ironically. Where is that "make art" button when we need it? I love rendering, but I hate promos. LOL
So how do we always complete what we start? Looking back to when I started, I believe that is a state of mind we set for ourselves. It's a commitment, so I am very careful when choosing what to create. It has to be something I can start and finish. In other words, it has to be something within my skill domain. If it's not and I want to do it, I have to train until it becomes presentable. I keep in mind what I want to achieve, so every little part is seen as one more step towards reaching it. And most of all, have FUN while doing it. Otherwise this may be a sign this is not for you. I love the chill of overcoming a challenge, and that's why when I get too comfortable, I try something more challenging. The last one were body morphs, and my current are divided between Python automation scripts for Poser, and face morphs. I am happy because they challenge me beyond my comfort zone.
Learning never ends, and I will never be good in everything. But I still love that feeling when you overcome something. That's what keeps me going.