How do I get my new cat to stop harassing my old cat?

I brought home a cat from the shelter I work at about a month ago. She was quarantined in a room for about two weeks until I could get her to a vet to get checked out and make sure she was not going to give my older cat something. I have had my older cat for about 8 or 9 years, and she has basically been my only cat during that time. I was hoping that the new cat could be a friend for my old cat while I'm working, but it doesn't appear to be working out that way. The new cat is much more friendly than the old cat, and she first started trying to go up to the old cat to be friends. My old cat didn't seem to want to have anything to do with her, so she now has started chasing her. When my old cat goes under the bed in my room (which is her spot and always has been), the new cat pursues her and all kitty hell breaks loose. I break up any fights they have as soon as I see or hear them, but I would like to know if anyone has any useful suggestions to get the new cat to stop chasing the old cat, and perhaps to get them to get along a bit better. I used the suggestions I found online and in a book that I read when introducing them (rubbing the new cat with a clean sock and leaving it with the old cat and vice versa, feeding old cat outside of the new cat's bedroom door, letting them see each other for a while but not come in direct contact until old cat stopped hissing), but it doesn't really seem to have done any good. I feel bad because I have to leave one of them locked up in a room while I am at work, but I also do not want them hurting one another while I am not there to separate them. Someone suggested I should restart the introduction process all over again, but I'm not sure which step to restart at. Any legitimate help would be greatly appreciated, as I just want a happy two cat home! Thanks in advance!