I used my sister's craft paper because I decided her unfinished project didn't matter

My sister held up the empty paper folder and asked where the blue sheets had gone. They were cut into decorations for my dinner table.

Her folder had been on the shelf for months. She'd started a paper model and hadn't returned to it. I assumed the remaining sheets were spare, although I knew they were hers and never asked. When she said she needed them to finish the model, my first response was, 'But you haven't touched that in ages.'

She said she hadn't realized there was a deadline on being allowed to keep materials for a project.

I can ask what would replace the paper. What I need help with is admitting the assumption behind taking it: I treated my immediate use as more important than her unfinished plan. How do I make amends without pretending I can simply buy something and decide the project is repaired?