Hi, and first, let me say how wonderful of you to adopt Belle. She’s very sweet looking.
Hay is more of an important food than a litter, so I would first suggest you switch it out a bit. Try a very soft litter, like plain Carefresh, so she can feel comfortable when she is in her litter pan. Also, Aspen shavings may appeal to her if you prefer an absorbant wood shaving. (Never pine shavings.) I imagine pelleted litter is a bit harsh for her to use.
If you put hay just in the corner for her to munch on, she may find that appealing as well. Try to mop up her pee with a small paper towel and set it in the litter pan, along with all of her poos. She needs to learn that the litter pan is the exclusive spot for that purpose. I’m assuming you have one end open so she can get into it without having to jump. (When Spockie lost the use of one back leg as a senior bunny, he was clearly unhappy but unable to drag himself into his litter pan.) Bunnies would prefer to have a clean habitat, so working with her should make some better arrangement possible.
The other point I want to make is that it takes about 2 weeks for a new bunny to become acclimated to a new home. Today’s only Thursday, and all things, including you and the smells and noises of your home, are very new to her. She may not be able to use a litter pan if she is scared by a sudden noise, for example. It will take her longer to maneuver into it, and she just may not have that much time when she goes. Be patient as she learns her surroundings and what is done where. That’s where the soaked paper towels will help her. They are also territorial, so she may be partly marking all her new home areas as “hers”. (And she’s happy about that, I’m sure!)
Give both of you time to start new routines and get established. Bunnies love routine, and adapt to them quickly. Once you have her digestion on a regular schedule, you both may be able to work out the litter pan use more easily. After or during her meals, watch her tail and if it comes up, you can help her by scooping her up gently and placing her in her litter pan, which will also add to her training.