Greetings,
I am studying Liszt's Sospiro, but it is very difficult.
I am studying it so I can upgrade my piano level, so in the future I can study a Chopin etude or another piece of Chopin.
What advice could you give me so I can aboard the piece (I have already done some musical analysis, but it is still difficult)?
As others stated, you are doing it wrong. Better to play simple pieces well that virtuoso pieces bad, and youtube is full of such ugly playing.
If you're not ready for Chopin Etudes, than you are not ready for Liszt Etudes (of any kind). They have kinda different problems, but to play both well you need to have really well technique.
It's good to play things one or sometimes two steps above your abilities, but not from few leagues above.
Also, I saw your propositions for winter, and besides that there's a lot of music there, but Sinfonias and Childrens corner are pieces for young studes, while c-minor Chopin nocturne is very serious piece, demanding power, strenght, goods octave and double-fingering technique, hand independence and musicality.
For me it looks like you cannot properly examine the difficulty of the piece, becuase there is whole world of difference between Chopin nocturne and others you wrote. I am not familiar with the Tango and Mozart sonata though, but whatever Mozart wrote, problems there are in musicality, not in fast fingers (well, from some level).
You're repertoire seems to be very widespread in what it demands from the pianist, as there are few levels of variance between the pieces.
Henle has a very good difficulty check, and Chopin nocturne is level 7, and sinfonias are 3/4 (depending which one).