Basically the title. I can't get better and play harder maps because I don't know how to read them anymore. Any map higher than 3* seems too difficult to me. Should I force on playing harder maps or practice more on the maps that I know I can pass more easily?

Just stick with getting familiar with patterns. Play maps that focus on a single pattern, maybe? That kind of sounds like you wouldn't be learning much though.. If you counter, let's say, a five note anchor on a stream section, then maybe you wouldn't know how to play that.

But you could still go with that approach. Or just play normally, having a mix of patterns in multiple maps. Try setting an accuracy goal, perhaps. and of course, focus on where you miss and how much misses you accumulate after a map.

When it comes to reading notes, there is a lot to say.

Firstly, note colour. There are skins that keep the note colours all the same, or change depending on if it is an LN, or use multiple colours to differentiate multiple lanes. Personally, I'm a half & half guy. With that note coloring, I can sometimes focus on both halves of the lanes like 2 two lanes instead of 1 four lane, if you get what I mean. I can only speak as a 4-key player though.

As far as reading notes goes, most people will say read somewhere near the middle, read where the notes are not located (the space between them) and learn the patterns.
Although it does very much come down to muscle memory, there's still a few tricks that can make note reading easier. Reading spaces in between notes is a huge one.
As well, I read notes as directions the notes are coming from for streams especially, if I should tap rightward (left to right) or vice versa because thats how the notes look to me, when to change direction, etc.
Sometimes, I may read clusters of notes as shapes and determine how to hit them, like a 21-2 jack would be a "triangle" for me.
If it's multiples notes at once (chords) you suck at hitting, I think I got over them by pretend playing, pretending I was playing a pattern without actually being in any game and it somehow with proper technique lead to proper reading. That can apply to more patterns. I think it was the result of me being bored of waiting for computers to load and open at school that I started fidgeting like that.

I don't know if I'm overcomplicating the simple act of reading notes. It's just me writing how I feel I got over reading certain patterns. The most important trick though I think is to simply look at the pattern. Turn on autoplay or watch a video and just look at that pattern because it's easier to read patterns when you're not playing, then play after. These are all just general suggestions and observations I made as I improved.

(Any edits made are for clarifying my sentence or improving its structure)

your at a stage where reading isnt mostly about high speed pattern recognition and heavy focus but more learning how a pattern looks as a whole and getting used to "grouping" notes together

you may have seen people play stuff like handstream and think to yourself "how does he even see that?" but trust me after a month of realizing how repetitive patterns are in stuff like handstream can get you'll eventually start seeing them way faster and sometimes thinking of the patterns name as you play it can help as it requires you to focus on what your doing and where your notes are moving instead of just reacting to it

at some point after you've mastered reading it should be good to try speed to build your skill and then try other reading skillsets like sv such as slowjam (people can find this really annoying to read but i highly recommend slowjams) and someday you might be able to read any pattern whether you like it or not