Ok I see what you're saying. But if you have learned something through practice, can't you validate it by researching / googling it? See what people are saying on stackoverflow, in blog posts, in books etc.? Also many university film and publish their lectures on youtube, i.e. Stanford Software Engineering course:
https://www.youtube.com/watch?v=KkMDCCdjyW8&list=PL3BD1325B3...
> But after years of good performance reviews, successful projects, and promotions, I started to finally allow myself to believe that I knew what I was doing.
Back to your original post, this kind of learning is FAR more valuable than anything you will learn in university. You have learned through practice and experience. I have probably interviewed at at ~20 different companies in the last 6 years and never once have I been asked a single question about my university degree, literally never. Nobody cares about them. They know that its the experience that counts, not the degree (though I'm not saying degrees are worthless either). In my experience I would say around 40% of the software developers I have worked with either do not have a degree at all or have a degree in a completed unrelated subject (i.e. Philosophy). The idea that one developer would discount another developer's idea because he doesn't have a degree in computer science is actually laughable.
Though I'm only saying all of that in order to tell you that a degree is not the answer to your imposter syndrome. The answer is to educate yourself, read books, experiment with different technologies and system architectures, read blog posts and discussions, watch conference talks, go to a conference, join slack channels and discuss topics with other developers. You should feel confident that you know what you're talking about because you have put the effort in to determine what the best solution is based off of research and practical experience, not because you paid hundreds of thousands of dollars (assuming your in the US) to have some guy tell you ideas about software engineering which are probably out of date anyway.