I'm still pushing myself to learn every day, but I constantly feel inundated by things I don't know and intimidated by people with amazing technical backgrounds. Others seem fine not knowing something, but for me it eats at my self-esteem, making it harder to be confident at work and difficult to enjoy the learning process.
How do you handle not knowing and cope with the impossibility of knowing everything?
What this means is that it makes no sense to feel guilty or ashamed about it, since it's not only an inescapable situation, it's the same situation that literally everyone is in.
What should cause shame and embarrassment is acting like (or believing that) you know something when you don't!
I'm a big believer in two things related to this:
1) Kettering's Principle of Intelligent Ignorance. The most important thing to know is what the limit of your knowledge is. We are all ignorant of more things that we have knowledge about, after all, and knowing what you're ignorant of helps keep you out of traps and enhances your credibility.
In other words, intelligent people recognize their ignorance. Unintelligent people are blind to their ignorance.
2) Discovering that you're wrong about something is a moment to be celebrated, not ashamed of. Being corrected means that I've just become a little less stupid. That's a great thing! As a corollary, acknowledging to others that you've been wrong enhances your credibility to others. It does not reduce it.
Professionally, the important thing is that you have a overview-level knowledge of most things, and an in-depth knowledge of the things that are important to your position (and positions that you're aiming to achieve). That's a much lighter lift than trying to be competent in everything.
IMHO Nothing will make you feel better, and also reinforce your understanding than helping someone else learn something you know. I would also look to someone higher up than yourself and reach out for help in learning something your having trouble with, most people will go the extra mile to share their knowledge for the reason I gave above.
What changed? I think the team got bigger, I got more responsibility, and I just got tired. I'm not the only one who can answer the question. There are other people who know (most) areas better than me, and they can answer the question. And I don't have the energy to be the guy who digs up all the answers. I've got other things I need to do with my time.
Answer honestly, ask for help, and learn.
You must realize that self confidence is not about thinking you are the best on what you do, but making sure that you are trying to do your best at every opportunity that you are given.
Once you assimilate that, you will understand that asking for help is sometimes the best way to do your best. And you will grow as a person with that.