I've had this feeling for sure when working in like... genres or styles or subject matter I'm extremely unfamiliar with. It feels a bit like learning a new language -- like being so used to making sentences that it's overwhelming to realise I'll need to learn the alphabet all over again.
Finding a good tutorial can be a big help, but sometimes not knowing where to start makes a "good tutorial" tough to pick out. So I feel like the slightly slower but low-resource method would be:
- start small
- find a reference of a single thing (like a human body for anatomy, or a single room for perspective) and try to draw that thing as close to the reference as you can.
- compare them, and see if you can figure out what isn't right in your version, and how to do it better next time.
- keep doing this
- find artists who are good at the thing you're trying to learn and copy their work, really examine how they handled it and try their methods for yourself
- try drawing the thing some -- stretch yourself so that you're just, for example, referencing a photo for a pose, and not directly copying every element of the exact reference
- you have now started learning the thing!! keep examining your work when you feel something isn't working, and comparing it to real things or good artists to figure out what you can change and improve. you can also get feedback from places like here on the forums to learn extra pointers you might have had trouble figuring out on your own.
Honestly a lot of times when I'm having this issue for stylistic or subject reasons, for me, looking at other artists' work and how they tackled it is a simple starting place to figure out what I need to learn. So like, when I started trying to work on a sci-fi based story, I started out by finding a blog full of robot illustrations and just copying my favourite ones, drawing them as closely as I could in my sketchbook. After I've drawn a few other peoples' robots, I have an idea of what sort of pieces I need to learn how to draw, and what sort of questions I need to answer about how my robots work, and I have a better starting point of Stuff To Look Up and Things To Practice.
(as an aside, if perspective is one you're working on now, this is a nice simple tutorial on approaching perspective without a lot of math, and one of my friends collected a bunch of my thoughts on drawing figures in perspective over here, in case either of those can help!)