Help! Cat introductions!

I have a cat named Miles who is a little less than a year old, amazing cat. He's very friendly, easy-going, wonderful to anyone and everyone, used to be good with new cats. However, one of my house-mates got a new cat, Annie, about a month ago who is about 2 years old. When she first got here, I was at a conference so Miles was being cat-sat at my boyfriend's house, so he came back to an already-claimed house (mistake #1). Then, we tried to see if they would magically get along for a couple weeks (mistake #2, they fought all the time and he just got scared and hid in his carrier all day and barely would eat, use the litterbox, etc). Then, we tried putting him just in my room, since we thought she would be the dominant cat since she's older and bigger, but he still was scared and they would fight through the door all the time (mistake #3). Now, we've separated them into upstairs and downstairs, with the door closed at the bottom of the stairs, but she still runs out occasionally and has been spraying the hallways. He growls a lot, is easily irritated still, and is now scared of any new cat he meets. It's been over a month and there's still no resolution. I realize we messed up the introductions, but we definitely need help! If there's anything you can think to do to undo everything that's been done or make things better, I'd really appreciate it! There's only a month and a half left of school, but I don't want anything else bad to happen or his behavior to get any worse.

Welcome to TCS! I would suggest a diffuser, Feliway is recommended by many here. I've never needed to use such, but I'm sure someone here can fill you in about it.

It's been 3 months and the new younger cat is still stalking and terrorizing our 11 year old cat. They are both females. We separate them while we are gone and that seems to help a little but it's so sad because my older cat doesn't sleep with me anymore. She hides. I've been told about the Feliway too and i'm going to try it. Tell me if your situation has gotten any better.