My solution is to write on paper that has already been used on one side, such as business letters I receive, or hard copy I make, that are no longer needed. I keep a pile of it, so I never need to write (by hand) on a fresh sheet of paper.
Then, the paper goes to a recycling facility (perhaps after shredding), so it isn't wasted even then, but has been given additional life before being converted back into more paper or cardboard. No new paper has been used.
If I need to continue writing when the paper is full, and I do not have any more with me, then I will turn the paper by 90° and write with a different colour ink, and on the original side as well.