Assuming we can use both lower- and uppercase letters (52 in total), with the ten digits and the underscore that gives us 63 characters to work with. A random 16-character combination of these gives us 95 bits of entropy (rounding down), which is secure enough by modern standards, at least for a home router.
Regardless, I understand the frustration of arbitrary limitations preventing you from choosing a secure password in a way that you’re comfortable with.
Just my opinion, but I’d say it’s okay to give up on many, even most things you try, but not all of them. In other words, go and try new things, it’s the only way you’ll learn what you enjoy and what you’re good at. And then stick to a select few of them.
There are also a few tips to be given on how to stay engaged with a subject. One of them: don’t stress it too hard. With something that’s meant to be pleasant, don’t push yourself to perfection until it becomes a chore. Also, if you want to achieve a more distant goal, it helps to find something to keep you going (look into how Duolingo, the language learning app, keeps people engaged).