How to learn 7k:
Play maps that are super easy until you can hit every single note without thinking.
Then look for maps that have 2 things: faster single notes, preferably short clumps of faster single notes; and 2 note chords.
Then look for maps that expose you to different combinations of single notes and 2 note or 3 note chords.
Slowly work your way up both in speed, and in handling chords.
Once you can handle chords and single notes in different patterns, that's when you should begin to focus on learning to handle LNs (long notes, holds).
When trying to learn LNs, remember: you need to both press and release the key on time to get a good accuracy rating on LNs.
When you start learning LNs, look for maps with single LNs being held, and other notes being played at the same time. LNs make reading much harder, so you need to start out simple, and get a feeling for what it feels like to play different notes (and later, chords) while holding a key down.
Learning 7k takes more time than 4k to get through the beginning phases of learning. Just reading 7k is harder than reading 4k. There are many more possible patterns that your brain has to learn.
It's also common to have periods of time where your improvement slows down, or stops for a while, so you shouldn't expect to always play at or near your peak skill.
And don't compare your learning speed to other players. Everyone ;earns differently. Everyone has different amounts of time to devote to the game, everyone has different map choices when they play, some people have natural talent for rhythm games, and some people do not. All of that affects the speed that someone learns at.
Talent: some people are just naturally going to pick up rhythm games and get good fast. Most people will not. Some people will be unlucky, and improve even slower than most people. It's not fair to anyone to compare your learning speed to other people's.
Mindset: you need to decide what part of this game you find the most rewarding. Are you playing just to improve and see how good you can get? Are you trying to rank up and get the most pp and highest rank you can get? Would you rather just toss on an easy map to mindlessly SS, or do you want to push yourself and see if you can even survive something that's way too hard for you?
If you decide you are taking things seriously and want to be competitive, you need to take some time and learn how to practice specific skills. If not, them you can basically play whatever. As long as you make sure not to avoid certain kinds of patterns, you should mostly see slow, steady improvement.