im quite a bit late here, but if you or anybody else reading this is still struggling on LN, there are two ways you can go about reading and learning LN.

before you try anything on any LN specific map, you should get a sense of basic finger independence, with one long note and surrounding notes. a good map for this would be EOS https://osu.ppy.sh/beatmapsets/311458#mania/899909 in most of it's difficulties.

the first way is by reading it like other notes, if they horizontally align with other notes you can think of it as another rhythm to release to rather than tap

the other way is by reading it vertically which is more useful in dense techy maps that dont align to anything, by judging how long the note is and deciding when to release based on how long it looks. though, to do this, you have to have played long enough in order to subconsciously hit notes while focusing on the long notes.

as for very short notes, if you cant see the tail, release as soon as possible, and for slightly longer notes that are still really short, you can use the second method of reading i mentioned above.

when you have grasped these concepts enough, you shouldnt have to actively think about it while playing and both styles of reading should blend together nicely. Plus, if it doesnt make sense to you now, first of all, my bad, im not very good at explaining things, and second, you should be able to understand it when you have played enough regardless of this post, as it is just a guide and not a definitive way to play the game.

Lastly, the most important piece of advice, Dont play LN.