Jay is a good friend of mine. We happened to meet around the time he was starting on software development. At that point, I was already knees deep on that world, so our friendship grew a bit around that.
For him, I was the one running at olympics speeds when he was barely learning to walk. So part of our relationship grew around that "mentorship". He often came with concerns, questions or ideas that I tried my best to answer.
At the very beginning his line of questioning was along the lines of "How do I make my software do X thing?". But as he started to get more skilled it shifted into "How do I make my software do Y thing correctly?" to the later "How do I make my software understandable for others?". This progression shows, at least for me, a great concern for his craft and is symptom of advancing towards seniority.
Now Jay has 6+ years of experience, and his latest concern is himself. He feels like he doesn't know enough. For this, I didn't had an actual answer but I know it is not strictly true. He might not go deep into the nitty gritty, but I know he has a pretty good grasp of things when it comes to software. This was just another case of Socratic paradox. The only thing I could offer him was putting his worries through a different angle.
Jay can drive his car. Does he know how to take apart a motor and put it back together? I don't think so. Does that make him a worse driver than anyone else? Of course not.
Jay could try and take his car's motor apart. First time he will be so worried about not losing a nut or bolt, that he will barely learn nor understand anything he is really doing. But if he keeps doing it again and again, Jay might start to worry less about the assembly part and start wondering "what is this part for?" or "can I interchange these two pieces?".
That is how I envision growth as a software developer. Learn how to get from A to B, commute every day, then start to worry what is in between.
Journey before destination.
TL;DR: As long as you keep aspiring to be better than yesterday, relax, you will be fine.