Don’t be afraid of empty spaces. There’s no need to clutter every flat surface with things. Choose a favorite piece or two to decorate your windowsill. Only keep things that you truly enjoy. Don’t keep them out of some obligation to the person who gave them to you. They are yours, you can do whatever you please with them.
Perhaps, in the process, you’ll find out that an empty windowsill is just right for you. You don’t know until you’ve tried. Clear out the clutter.