my cat has gone insane

Help! We adopted a stray cat in May and my current female cat, Shenzi, immediately hated the new male cat, we name Kiki. They are both nuetered. People told us to give them several months to get used to each other, but that hasn't worked. Shenzi constantly hisses and growls at Kiki. First she growled when he was in the same room, now she growls constantly, even when he isn't even in the house. Since we decided to let Kiki be an indoor/outdoor cat, my husband thought it would be a good idea to let Shenzi also be an indoor/outdoor cat. Well, that sort of backfired. Now we don't see Shenzi for days at a time. She only comes around to eat a few bites of food and then wants to leave right away. She has also begun to growl and hiss at us all the time. I don't know what to do. I wanted to have two cats that could keep each other company, but now ended up with a nice cat, Kiki and a crazy outdoor cat that doesn't want to be around anymore. I'm ready to try and adopt her out, but I don't think anyone would take her. (She never had a very nice personality, and now she's psycho.) I don't want Kiki to be an only cat. We want to get a dog, too. We raised Shenzi from a kitten so it's hard to think of getting rid of her. Help!

take her to a vet first to rule out medical problems , cats can get aggresive when they are sick or in pain. if she is fine then i think you should consider a cat behaviorist (sp?).

goodluck and please don't give up on her