I have always thought "Moonlight Sonata" from Beethoven was/is a masterpiece of sound. The depth of emotion I hear in it is breathtaking. Now I'm no piano player, but I can at least count lines and spaces on a staff. I determined that I was going to learn to play it. It took me months of sitting at a 100 year old upright and figuring out what C# minor is supposed to sound like, but I got through it. I still remember the day when I started at the beginning and played it completely to the end. All that emotion, those dynamics, the passion and crescendo, all came out of my fingers! What a feeling of accomplishment.
I've done similar things on guitar, like "Black Magic Woman" (the Santana version), but I'm slightly more skilled on guitar so it wasn't so high a mountain to climb. To my point:
1. Pick something you really want to learn. It doesn't matter how hard it is, it only matters how badly you want to learn it.
2. Set yourself up for success. Find the right time, the right tools, and the right location and go to work.
3. Allow yourself to have success along the way. Smile when you get that first phrase, or riff, or verse, or whatever. When you get each success, enjoy it. Forget that you have an elephant in front of you to eat. Just focus on that bite that is done and you are better for learning it. Then take out the next one.
4. Finishing the song is not the goal. The only goal is the next phrase. Just attach it to the phrases that came before. Like sentences become paragraphs and paragraphs become stories, phrases become verses, and verses become songs.
5. When you finish you will find that your complete skill set has improved. Your timing, your dexterity, your ear, everything. When you can play that challenging song like you want to play it, everything had to improve to get you there.
It isn't practice, it's sweating pearls.
I can't play much of "Moonlight Sonata" any more. It's been too many years. I'd have to relearn it, but now at least I know I can.