How do you feel confident in content?

I know classroom management and a lot of this stuff just comes with experience and time, but I can’t help but feel worried. And some people even say knowing the content doesn’t matter. I’ve been doing observations for two semesters now and only just began studying for the CST. I’m planning on being a biology teacher and just really worried since I keep forgetting things. I remember when I hear them, but more often than not I’m clueless when I watch my preceptor, and it makes me nervous.

How did you become confident in your content knowledge? And any recommendations on how I can study and, by extension, feel like I know this?