I have a cat that spends the entire time indoors because he is white and gets cancer. I know what they are like sometimes. My suggestions to help with depression and boredom are

1. Plenty of complicated climbing frames or one big one with lots to do with it! Lots of hanging things, lots of cubby places to hide - will keep cat amused for ages ( and you! )

2. A few pots of cat grass and even a pot of ferns near them as well so she can smell and eat green things and hide in the ferns

3. Music or TV playing softly while you are out. Will keep her company and help her get used to the noises inside.

4. Catnip. Fresh catnip is best, it does grow in pots and you can then make your own toys or sprinkle it in places she likes to sleep, will keep her calm and relaxed

5. Bones to gnaw on like raw chicken necks ( all 4 of my cats love them and they love the crunching! ). Great for her tummy and teeth. I think indoor cats miss eating a lot of the grass and stuff and the bones help clear out the fur from their insides...makes kitty much happier

OK, those are my suggestions, hope they give you an idea :)

Is it a single cat household? Have you thought about getting a kitten?

It's always said that 2 cats are easier to take care of than one. Might give the cat a pal to play with or he/she make take on a parenting role.

It's hard for cats to get used to being indoors when they're used to being out/in. There's no easy way to do it. I once had a cat, that ambushed me every time opened the door. Once I tried to chase her to get her back, and she wanted out so bad she ran, and sadly ran straight into a car. She left at 7:05pm and was called in at 7:10 being hit by a car.

I applaud you for keeping your cat in. It's dangerous out there.

Cats, at least from my experience, are creatures of habit when it comes to their environment. Your kitty may just need some time to adjust.

Perhaps a familiar element from the outdoors such as cat grass (we use vita greens, http://www.petco.com/product/5430/Four-Paws-Vita-Greens.aspx) would help your feline acclimate.

I would also suggest a variety of different types of toys and regular playtime with someone in your household. My cat's two favorite toys are a loco laser (http://www.walmart.com/catalog/product.do?product_id=10804640) and a round stuffed dog that we found at a local dollar store, but I try to make sure she has plenty of other toys to keep her busy.

My cat is a girl as well. She likes a lot of attention and petting (on her terms, of course). Yours may be wanting attention as well although I would be cautious about petting if she is aggressive.

This sounds a lot like my cat's experience. She's always been an indoor cat, but a few years since we had her, my parents also got a dog. They got together basically like you'd stereotypically imagine cats and dogs to get along -- i.e. not well. There was lots of fighting, barking, chasing, clawing -- often really violent. She was a small dog, but still. And then a few years later, my grandmother moved in and brought HER cat, and things just escalated.

Honestly, the only advice I have is to give your cat lots of love and attention so she doesn't feel left out. Her home probably doesn't feel safe anymore, but you're a constant, and ignoring her won't help at all. If she has favorite toys, having those on hand can help (my cat never much liked most toys, but maybe yours does.) And if it's at all possible -- it might not be -- it can be helpful to avoid more major changes.

Hope some of this works out for you and your cat!