If you're concerned about ruining the sketchbook, I have a few suggestions that may help:
-You can get one that is relatively cheap that you use just for practicing in. Make it a rule for yourself to never pull out the page if you hate it, simply use the back for more practice, erase what you don't like, or use the rest of the blank page and start over, once you like it, redraw it better on a clean page, or leave it as practice.
You can make finished pieces in a nicer/different sketchbook, or on a loose page and keep it in a portfolio.
-You can segment off the back of the sketchbook for practice. You can use the last third of the pages for practice and make a tab on the page it starts you can find your practice section easily. That way, all of your nice work stays in the front, and you don't buy a second sketchbook.
-You can get a sketchbook with rings or perforated edges to save the binding. If the sketchbook has pages that are meant to be torn out, it won't ruin the binding. If you don't want to rebind the one you have, you can draw in the sketchbook, and even if you like it, pull it out (carefully, or cut it if possible) and save it in a portfolio. Then your sketchbook is like a holder for loose paper.
It seems that you have some anxiety about how your work comes out. If you keep yourself from pulling out the pages, and embrace the mistakes, you can use your sketchbook to gauge your improvement over time.
Anything you like and then "mess up" can still be worked into something good, and if you give up hope on it, save it anyway. You might want to draw something again someday, and you can refer back to the practice/old work and reproduce the elements of it that you like.
I never draw on the first few pages of a sketchbook, for some reason, I always leave at least the very first page blank. I guess we all have our idiosyncrasies about new sketchbooks, haha.
Also, don't feel bad about not finishing a sketchbook!
You never know, years from now you could come across it and flip through your old work. Next thing you know, you're inspired to draw again and you can work in the blank pages. I've done it, I also still have a lot of sketchbooks with blank pages.
I hope I helped!
Stay creative!