It doesn't really matter how well you know something, unless you are being held in regard. You learn throughout the years that knowledge is basically an organization of experiences, and somehow you learn to let go of being stubborn in learning something in details.
What I am arguing is simple: You don't have to know everything and the best way to learn something, but you have to understand how you approach learning.
When we talk about approach to learning, it in a lot of ways connects your approach to that of other aspects in life. I believe that you approach everything as if it's new from the very start, and you learn how to deal with it. That is to say, your approach to learning is in fact so fundamental in you that it is the connection cable to the window of the world and how you observe it.
Do you:
a) use feelings to make sense?
b) execute then only to learn from experience?
c) organize the information and understand the connection?
d) learn with verbal aid, i.e. talking to yourself or others?
e) learn with non-verbal aid, i.e. walking around or other physical movements?
...
etc.
Of course, when you learn different things, you approach them with different methods. But what I found out is that there's a heavily repetitive pattern occurring over and over again, individual-wise. For me I usually need time to organize stuff, establish connections and applications, and talk to people to materialize or iterate over what I just learned.
It's ultimately important for you to understand how you approach something new. Once you grasp, more or less, the way you approach learning in general, you will find out what method works for you and what doesn't, and it is actually a tool for you to regain that good old confidence, telling yourself: yeah it's just not the way for me; and more often it offers you a fresh insight on how to push yourself back into the career / family path that you want but somehow got off course. When it's off course it means you need to learn to fix it, and when it comes to learning, it's about how you approach the grave matters.
It saves my ass a few times already. I hope this helps the rest of you who are somehow stuck in something in your life. Good luck!