I'm sorry to hear you're having problems with your new girl. Try just sitting on the floor near the bed and try to engage her in some play, with one of those toys with feathers, fringe or ribbons on a stick, or make one yourself. Drag it around in front of you, but don't poke it at her. When you do look at her, do not stare and give eye contact--cats interpret this as a threatening gesture particularly if they don't know you, but do slow blinking
then look away; this is calming for cats and says: "relax, I'm not a threat and mean no harm". Catnip is calming as well, tho not all cats respond to it. Rub a little catnip on a toy mouse and see if she'll come near you to play with it. Cats have a keen sense of smell so will smell the catnip as soon as you enter the room. Ignore her hissing, and keep talking to her in a quiet singsong voice and remain calm, and offer treats a few times throughout the day, and hopefully she'll be looking forward to your visits, either to give her treats or some new interesting play toy, like a foil ball, or plastic pull off a frozen juice can.