I just brutalized myself for several days trying the first page of Liszt’s arrangement of the first movement of “Eroica”, and ended up sounding like a complete train wreck. Go me.
I then play my own stuff, and I can do things I cannot do when they’re in someone else’s music.
Frankly, playing one’s own stuff is way, way easier than playing someone else’s in terms of having a prayer in hell of keeping up. They really are two very different skills. And the more I do the first, the more I simply can’t manage the second. Oh my GOD you have never heard anything worse. I’m serious.