I agree with Gill, you may have to go back and start from scratch and reintroduce, theres still no guarantees that that will work, but leaving them to sort it out between them is defintely not the way to go. Ideally intros should be done slowly, with the cats not even meeting face to face for a while, so that each cat has a chance to get used to the scents and sounds of the other, then face to face meetings phased in. That way, you stand a good chance of them accepting each other (doesnt always work though!) If you do the intros too soon and things go wrong right from the start its very hard, as you have found, to get back on track.
I have been successful with slow intros over the years, until I adopted Jasper (together with his lifelong friend Zeta) about 18 months ago, despite doing slow, careful intros, Jasper and long time resident Harry will never see eye to eye, although there is now a level of tolerance - just, and I would certainly intervene if things escalated into anything more than a growl or hiss. But thats different from what you are experiencing -with Jasper and Harry there is no bullying or constant hounding of one or the other - they just loathe each other in equal measures!
The slow intro method would be worth at least trying, because if you do and it works, then thats great, if it still doesnt do the trick, then you'll know its probably never going to work and rehoming may be the only option.