From my personal experience, transitions can take from 3 days to 3 weeks. So don't worry, things WILL get better.

Meantime, you can do things to ease them both along. Make a fuss of, and talk to, your girl first. So she knows she is still number 1. THEN talk to the new little boy. When you feed them, put food in HER dish first, then in his. Pick HER up, snuggle and talk to her, put her down and THEN pick him up and snuggle and talk to him. the pick her up again, so she gets his scent off of you.

Play time is also a good time to get them together. Feathers on a stick, or something on a string. Playing with toys will help them focus on the toy and on you, not each other.

Initially, I would not leave them alone together while you are out of the house, at work and at school. But as long as you are around, let them roam and wander. He will want to explore his new digs. Just keep the water spray bottle handy. When you see her getting into pounce position, talk to her, approach her, clap your hands, distract her from the attack reassure her she is your girl. And squirt if necessary.

They are young, this own't take long. Let us know how things develop!