It's okay if you can't beat the demo level, platformers are meant to be hard, so if you can't beat it, that's good! Personally, the flying strawberry screen is the hardest part of the demo level.
I wouldn't say that platformers are "meant to be hard" but Celeste does seem to be. And yeah, the demo level has some pretty bad screens.
Don't know if you could use this instead of the demo level already there, but I made this.