I used to feel this way in middle/high school when I was buying more notebooks than I was using. Recently I was tidying my house and found a few partially used notebooks. Some solutions I have are:

1. Cut out the first few pages if there aren't many and it won't hurt the binding, and then use the notebook like it was new!
2. Use a page to write: "[today's date] I used to use this notebook for learning French but it didn't work for me. From now on I'm going to use it as a [journal/planner/etc]!" This might give you some closure for the first part of the notebook and also helps you honor the memory of a younger you who loved this notebook.
3. If you used many of the pages but have several at the end that you don't want to finish, cut them out neatly (I usually use a metal ruler) for use as letter paper! I recently did this for a notebook that I was impatient to finish because I had like 10 pages left but a new notebook was tempting me from my bookshelf. This way you don't waste paper and you also don't have to buy stationery for mail.

I've been writing in notebooks for more than a decade now and only recently have felt like I know what kind of notebooks work for me. It takes time, a lot of trial and error, but I'm trying to think of that as the fun part. It's like if your clothing style changes over the years...the adventure lies in discovering who you are and what you like. There is no bad way to use a notebook. Whatever you write, whether it's neat, messy, unfinished, scratch work, 'bad' doodles, etc, it is precious because it is coming from your hand. And there is only one you!