I have been playing Geometry Dash for 5 years. My hardest was Nine Circles a year ago and today I have been improving fast. I have beaten multiple extreme demons and I am about to beat Bloodbath, a level I never thought I would be able to beat in my life.
How did this happen? Well, here are the reasons I've noticed why it took me 5 years to finally beat my first extremes, and if you are in the same situation as I was in, maybe you could apply the tips I'll share with you today. Maybe in a few weeks, you will beat your first extreme demon!
First of all, don't just beat easy demons even if you think that you aren't capable of beating harder levels, like, how are you going to know if you improved if you're not trying anything difficult? I suggest that you try to do some runs on extreme demons. You're going to be surprised at what you're capable of achieving. Always persevere and push your GD skills as much as possible. For example, try and get to the ball in Sonic Wave or beat the last part in Bloodbath.
Secondly, and this was very crucial to my improvement, the demon list team might hate me for this but if you're stuck on 60hz, use the FPS Bypass hack. It unlocks your true potential. You might think that this is cheating but in my opinion, if you are not able to afford a high refresh rate monitor, there is no reason to keep yourself at a disadvantage. RobTop even said that he is adding FPS Bypass into the game in 2.2!
And finally, beat a bunch of challenges! If you're bad at the wave, try some wave challenges. Always practice game modes you're bad at or you will end up with a very unbalanced skillset. I am looking at you, challenge players. What I like to do whenever I beat a wave challenge is that I increase the speed by 10%, so 1.1x up to 2.0x. Because of that, I went from being terrible at the wave to it being my strongest game mode in Geometry Dash.
I hope you thought that these tips were helpful. Beating new hardests back to back after applying these tips has made the game a lot more fun for me!