What should I do next??

I have a previos post about introducing my Resident, Bella to my New kitty, Trixie. Thanks for all the advice about keeping them separated --I will of course do this for as long as it takes. Its been 6 days and I have been trading scents for 3 days. They were both taking to scents well -- Bella sniffed Trix's blanket in her territory for a good 10 min and seemed ok with it and continued on as normal. Trix rubs on Bella's socks and purrs like nuts. Trix seems to be adjusting well to me (fell asleep in my arms) and Bella has settled too (yea! Feliway!)
so last night I sat at the door separating them talking at Trix through the door. They were both there and finally got a quick glimpse of each other before I could close door. Bella was curious -- sniffed and sniffed but it must have scared or?? Trix cuz she HISSED big time. Bella sorta was taken aback cuz she seemed very friendly but backed off and just looked at me like 'wow'. I took Bella further back from the door and pet her, talked and said its ok.
Then I went into Trix room with door shut. Under the crack in the door I could see Bella back at the other side sniffing under the door. Trix just wantd loves from me, but when she sniffed back under the door, this time Bella HISSED big time. So I pulled Trix away and made sure she was ok before leaving.
Went out and pickedup Bella (first time I have EVER heard her Hiss or be upset - it upset me!!) and she whimpered at me and was so worried. I spent the rest of hte night upset and not sure what to do.. Did I just push it too fast? Was this a bad sign they will not like each other? is Hissin like this normal or were they jealous? I noticed the one hissing was the one without me in the room.. Its only been 5 days and I am exhausted from worrying about this which isnt good cuz I am sure they can feel the stress on me. AM I doing it right???
thanks for help

You're doing fine, but they aren't ready yet to spend time together. I think you can proceed with the next step, that is swapping territories. With your resident cat confined to one room, you can let the new cat explore the house, and mark it up with her scent. Then put her back and let the resident cat out. When that goes well, put the new cat by herself in a different room, and let the resident cat explore the new cat's room. The purpose of this procedure is to intermingle scents in each cat's territory. Don't ever let the cats actually SEE each other.

After you've done this for a couple days, and if it looks like things are going well, then you can try having them interact through a small crack in the door, feeding them both treats to give them a pleasant association with being near the other cat, and also to divert them from expressing any hostility. You can also feed them on opposite sides of the door.

There, that's two steps. Good luck!!

I found putting a childs gate up to the door and letting them look and sniff each other helped also. Only done when I was there watching. All my other cats where very curious about the new kitten and wanted to watch her.When she came right up to the gate they would hiss and back up. By the 4th day of bringing the new kitten into the house. She had been accepted and runs freely around the house with the 3 resident cats.
Not a fight but my female siamese stills isn't acting like she used to she would rather new kitty wasn't here. She wouldn't hurt her but she just seems abit miffed about her being here.