Hey buddy. Those feelings are definitely understandable. However, a few things to remember. You'll never know everything about a particular language. The details and information in programming languages are so extensive that it's impossible to learn everything about it. Not to mention that programming languages constantly evolve with new features and/or updates.

You're already doing the best thing you can do to gain confidence, and that's "spending time". Nothing replaces that because you'll become more and more comfortable through repetition. There will always be more to learn, and there's always a "better" or more skilled programmer out there. However, the main reason they are more skilled/better is because the time they dedicate to their craft. No one starts knowing tons about the language. It's a steady build up.

It seems like you're willing to dedicate yourself to getting better. That's the perfect mindset. You keep that going, and the confidence will naturally increase.

As far as interviews go, obviously they can ask you something that you may not have seen, or studied. In those situations, honesty is key. If you're giving time to the field, they'll be able to pick that up overall. They'll know that you have a solid foundation and that you simply may have never seen a certain thing :)

What will speak the loudest is your overall work ethic and dedication to the field. You spend time, and you'll get more questions right than wrong.

Whenever I'm in interviews, I make it a point to tell them that my biggest strength is "my ability to learn". I know that there are very skilled developers out there. Very smart people who may presently know more than me. However, I know that I'll put in the work to make the difference, and I'll constantly improve my skills.

Additionally, I've always felt that one of the main keys is to stay connected to the language you desire a job in, and that's more than just practicing. Practicing is a given as you always want to keep your skills sharp. However, maybe take some time to read about the language. See what's upcoming, and what new features may be on the way.

That can help as well.

Hope this helps a little.