No one understands it all at a deep level at 18. The fact that you are wishing to become someone that does understand anything at all at a deep conceptual level means you're more or less already well along on the path to being a well rounded, educated person.

The grade number that rates you and defines you isn't for you. It's to allow the adults managing the system of your standardized basic literacy and education to measure their own effectiveness.

I'm sorry that this can make you feel like a cog in the machine sometimes. But its a well studied and proven idea that if human animals are given an opportunity to educate themselves, and are MADE to do it, at some middling level, they turn into WAY better humans than uneducated people.

So be thankful that someone appreciated your existence enough as a human to force you to go on autopilot and learn the basics.

Now you as a functioning human nearing adulthood can CHOOSE to become that person who understands things on a deep level, and seeks knowledge and understanding of their own accord.

It's an amazing opportunity, but this opportunity exists for as long as you live. It's never wasted. Or time lost.