And breathe!
I can see how this is stressful for everyone in the home. It's not fair on Bella and Mouse and as a result - you are unhappy and stressed.
Have you done a proper introduction process between the two cats? Please search on this site on how to do it properly. It takes time - it took me 5 weeks to introduce two kittens to an adult cat. I also relied heavily on the the Jackson Galaxy method.
Here is a record of my journey:
viewtopic.php?f=19&t=9382&p=62978&hilit=kellen#p62978
So imagine your parents suddenly brought home an adopted sister for you to play with. You has no warning, no inkling. You had to share all your resources and space with the strange girl. How would you react? This is the same for cats. They need a gentle and patient introduction to each other.
Firstly, make sure Bella has her own litter box. It's unpleasant to have it in your room, but it is necessary until Bella can leave the room. Also ensure Mouse has his own box too.
Then try scent swapping. Rub socks and blankets on both cats and introduce the scents to each other. Do this everyday for about a week.
Then switch to the feeding either side of the closed door process. This is detailed on Jackson Galaxy's website and youtube videos. Do this for a week. Stop free feeding and have two set meal times a day. This way they will have to eat. Keep the bowls say two metres apart from either side of a closed door and then gradually bring them forward every meal time. If a cat refuses to eat, put their bowl back to where they last felt comfortable and carry on with that for a few days.
Try these two things above and let us know how you get on.
You can also let Bella around the house when Mouse is not around so she can leave her scent around and take in his scent.
I highly recommend Feliway Friends plug in too. Have one in Bella's room and one where Mouse spends a lot of his time.