Professional developer with 5+ years experience here
and I still can’t remember how to use grid or flex box without having a cheat sheet open in a browser tab to the side 
This is totally normal in my opinion. The simple answer is you have to live through these struggles. It’s a sign that you’re spending more time learning through tutorials and guided content as apposed to just building stuff.
If that simple website took you a long time, document how long it took. Start a fresh project and see how long it takes you to build the exact same site again.
You really just have to increase the amount of learning you do “off rails”. Tutorials are holding your hand too much and so you’re not experiencing the pain of making decisions and figuring things out on your own.
This is just a reality for most people. A small select minority manage to find that first entry-level job within the first year but there’s a complex combination of complimentary background, raw talent, less distractions in their life, more available time to dedicate toward learning, better at learning in general, luck, different opportunities etc…
You gotta think, some people go the post-secondary route to get into the industry and they’re looking at 2-4 years of school (sometimes more) plus if they don’t get into some sort of internship program provided through the school they still have to jump through all the same hoops self-taught developers have to jump through.
It can be a long journey no matter the path you take.
It was roughly 5 years for me but I wasn’t consistent with what I was learning or how much I was learning throughout that. Once I knew for sure I wanted to be in web, it took roughly 3 months of insane focus to get the first job and it was 90% luck. I also had almost 5 years of learning how to program in other tech stacks before I decided to focus my efforts on web, as well as had dabbled in making websites and working with web technologies over the years.
Yes. I find myself to be a naturally deep learner. Which means I learn slowly. I’ve learned to overcome this a bit over the last half-decade or so but I really struggle to pick up new concepts quickly if I don’t understand all the moving parts.
The single thing that has helped me the most is to switch my focus to building as much as I can while knowing as little as possible. Then afterwards I scratch that itch to know more and dive deeper but only after I’ve built something real with whatever it is I’m learning.
I would never take a course on something I want to learn without building something with it first.
Good luck with your journey!
Edit
Oh another useful tip too is this concept called “code katas”. Basically the idea is to isolate a task that you want to get better at and practice that one thing over and over again.
So an example could be that you want to get faster at making a nav bar. So you could take a web site that you’ve already built and just delete the nav bar. Practice adding a nav bar to that website over and over again, building up your speed, and experimenting with different approaches. Over time you’ll feel extremely confident at that one task.
And you can try that approach with anything you want to get better at. Like practicing your scales if you’re a musician.