A sketchbook is a place to be terrible in private. Everybody who got good had one.

Skill requires a large quantity of bad attempts, and the bad attempts need somewhere to live that is not public. A private space lowers the cost of every individual page, which means you make more of them, which is the entire mechanism. Nobody arrives at good work without a pile of unseen worse work behind it.

Keep one book that nobody else gets to open and put the failures in it deliberately. Do not tear pages out. Filling it badly is the objective, and the finished pieces you eventually show will be drawn from what happened in there.