Well ... you didn't properly introduce them and now you're paying the
price. It is fixable, but it will take some time. It's not the cats or
the dogs fault - don't blame either of them - they're just doing what
comes naturally. Go to a pet store and buy a baby gate - something that
the dog can't get past and give the kitty a sanctuary room beyond the
gate for a few days with food and a box where doggie can't go. Kitty
will be able to jump over the fence, but the dog won't. They will see
each other, smell and each other and react to each other, but not able
to interact unless kitty wants to. This will calm the cat down. Give
each some private love every day and feed them right by the gate where
they can see each other. If you do this for a while - they should both
start to calm down. Then try to take off the gate for an hour or so
when you are home. If they fight again, put it right back up. If they
can get through an hour or so without fighting, then put the gate back
up anyway, but try longer and longer intervals when you are home to
supervise. Once they can ignore each other for a few hours - you will
be pretty much out of the woods. I know it's frustrating, but don't
give up and keep working on it. In a few months, all should be well...