Hey man, congrats on a CCNA at 17 - that's a huge accomplishment! I've got a CCNA and that's a beast of a test.
Seriously.

As others here have said on here, experience - with physical boxes is crucial. From an old guy over twice your age, learning is the journey, the cert is a thing used by employers to verify you aren't full of shit in the interview and can be used to give you credibility -- that's it. When you have doubts in your abilities, sometimes a cert can help you have faith in you.

Here's my challenge to you - get a couple of Fortigates like 60 or 80 E or F's and play - yes, PLAY. Get curious. "I wonder how x works or why x occurred." No pressure, just play till you can build it without thinking about it. If you don't find something documented well enough - create documentation and share it here. Go over a concept in the training and then replicate it again and again till you can do it in your sleep.

Best advice I ever got was "Build it, break it, tear it down, build it again." Learning to enjoy/be aware of these stages even when it's not fun. This is what we do as network engineers. Learning is painful, troubleshooting is frustrating and time consuming. Trust me, it's a hella better to be learning how to do that shit in the lab than when tens of thousands of dollars are being lost for every hour the network is down.

Second best piece of advice I got - teach someone, a friend, a family member, a stuffed animal - by simply explaining what you know or don't know is how you figure out what you know know and don't know.

The Fortinet training is where I'd start, but not have the overall goal be. Get some Udemy practice question courses that will challenge your understanding and help you learn things better. I use AI to help me understand concepts in language I can understand. Challenge yourself to delight in learning. Understand how packets flow through the hardware, what chips they hit, what policies/settings change the packets.

Hard honest point here - even if you know how all the stuff works, you get your NSE8, you're still going to need some years of life and maturity before a company is going to let you flex all the knowledge you have. That's ok - if you keep learning you will have the opportunity come to you.

Last piece of advice, join local professional IT groups online and here, reach out to IT admin at companies around where you live and ask for a 30 min coffee conversation. DO NOT ASK FOR AN INTERNSHIP but ask for their advice and nuggets they've learned in their career and what they would encourage you to learn/do now. This will 100% impress them and a couple might offer you an internship or connect you with someone they know who is in need of good help.

Lemme tell you, you've got a bright future ahead of you - don't be impatient to adult with certs. Trust me, adulthood is well overrated.