How long would you say it took you before getting a fundamental understanding?

I would say years, as with any complex activity.

I'm still forgetting things I learned 3 or even 4 times like how to do a for each loop.

You can forget in 2 different ways:

1. Forget how to use something, so you need to look how to do it.
2. Forget that something exists, so you cannot even look for it because you are not aware it's a possibility.

You will forget-1 everything which you don't use on a daily basis. That's what internet is for. Forgetting in the 2-nd sense is much more rare and you should do something if that's the case.

all of it feels too advanced and I get lost on how to begin

This is a bias most of us have, you overlook how easy is for you to do things that previously were impossible and focus on how hard are the things you still don't know how to do. And computing is so complex right now that there always be "infinite" things you don't know.

Try showing what you know to someone who doesn't know how to code and you will get an idea of how much you have learnt :).

Anyway, I don't really have good advice :/, just wanted to confirm that what you feel is expected. Good luck!