mattered hair

my cat has awful mattered hair... how do I relieve her of the pain and help prevent this from happening again?

She should be taken to the vet or a good groomer and shaved. It is the only way. Please don't try to do it yourself. Matted hair is very painful and she may even need to be medicated while it is being done.
Makes sure she stays warm afterwards. Without her coat, she'll be pretty cold.
And when it grows back in, get into the habit of brushing her every day, as important as brushing your teeth. It may be tricky at the beginning bcause she'll be nervous, but they usually end up by liking it.

I second those great suggestions Badger :thumbs up

If you tried to brush it out as it is now it would cause untold pain & she would never allow you to brush her in the future - a shave by a vet or groomer is your only option.

Start with short brushing sessions after the shave I've found the Cat Zoom Groom by Kong (http://www.amazon.com/gp/product/images/B00009ZJ0Q/ref=dp_image_0?ie=UTF8&n=284507&s=kitchen) to be great for brushing, it massages at the same time & is well appreciated by almost all cats too :)

Yes, sounds like your cat will likely need shaving. You could ask your vet to recommend a good groomer for cats. Many of them seem to focus more on dogs, so finding a good one is important. Of course, if your vet will do it, that would be even better, as the cat probably will need to be put under anesthetic. What are you feeding your cat? I have found certain brands of food to be better than others for my cat who tends to get mats. It may be you need to switch to a different brand or supplement your cats food to help prevent mats.