I'm in my 6th year of a (hopefully soon-to-be-completed) doctorate, and my experience has been very similar. I started the degree because I wanted to learn more about a particular field and develop the related technical skills. I couldn't think of a job where I could get the same benefits. I've learned a ton, but have also seen that while I have certain strengths, there are others who are smarter, more motivated, and frankly have different priorities than I do. Like you, I'm now coming to terms with it, but I have a better idea of what strengths I do have and am working towards putting them to good use.