I, too, am self-taught. As a tyke, the closest I came to sewing was playing on the floor with the wooden thread spools while my Mom was working on one of her projects. In eighth-grade Home Ec we made a couple minor items (I remember a draw-string bag), but nothing of any consequence. For the most part, stuff fits me pretty well off the shelf, so I did not have any personal motivation to try anything.
Then I got married, and shortly before the birth of our first daughter, my wife wanted to try making a quilt. We went down to the local sewing store and came home with a Bernette 50. The quilt took a lot longer than expected, but I learned a lot about making straight seams.
Several years, and a new house, later we needed new window coverings. After deciding on roman blinds and then recovering from the sticker shock that came from browsing for commercially made ones, I bought an eBook on roman blinds and dug the machine out of storage. Ten blinds later, I was feeling very familiar with the machine.
After this much success, I decided to attempt making my wife a skirt. She likes longer skirts, and at the time there were not many available. Plus, her ratio between hip and waist is significantly out of the normal range for RTW, so finding things that fit well is a real challenge. So, we found some fabric that she liked, and bought a pattern that she thought pretty and I put it together. It turned out okay, but not having any experience in altering meant that it didn't fit much better than the RTW stuff. Then, other stuff (like finishing a basement) intervened.
Recently, my youngest daughter (4-1/2) decided that long skirts are her favorite clothes. She must have inherited that from her Mom. Anyway, I decided to make her a half-circle skirt to wear. Having learned something from my previous experience, I made a muslin first to check on fit and construction details. I made a few hideous errors, but she seized upon the final muslin as a "play skirt" with great enthusiasm.
Thus encouraged, I made another out of the pink fabric I had chosen. Fixing as many of the muslin mistakes as I could, I ended up with something that I actually thought looked okay. My daughter loved it, and insisted on wearing it two straight days, finally crying when it was taken away to be washed. Her elder sister was immediately jealous, requesting one in brown!
In the end, I am just finishing my fifth skirt, with two more to go. Along the way, I have looked up various things on the web to clarify issues I was facing. I have reference Kate's page numerous times as I have tried to make each iteration a learning experience, adding something new to my body of knowledge.
Along the way, I purchased a kit from Sure-Fit Designs (now, sadly, out of business) to make a custom dress, blouse or skirt pattern. Having followed the steps in the instructions we now have a muslin for a skirt that actually fits my wife like a skirt should fit! The next step is to make the real thing.
So, to reiterate the postings of others: success is the greatest motivator for continued attempts, and making a given project more than once allows you to refine your expertise of the techniques needed by that project.