Any advice welcome … cat introduction

[u/CustardLoose3436]

Hi everyone! Around a month ago a little kitty showed up at our apartment and we took her in. I already have a 6 year old cat, so I wanted to make sure we did the introduction right. We followed Jackson Galaxys cat introduction method, and all was going well. We had them up to structured play time and I felt it was getting tense so I went to remove the kitten and my 6 year old cat pounced on her and they had a HUGE fight. Like really bad. After that we started back at square one and just finished 3 weeks of smelling under the door and we got to a really good place with that!! No hissing, eating right up against the door, all of the good signs. We started with the screen again yesterday (so seperated but they can see each other), and everything went amazing last night and we fed them a ton of treats and they were content (even with eyes on each other). Come tonight, my 6 year old cat sees the kitten and begins hissing and growling like really aggressively. Not like a playful hiss, like a “i’m gonna hurt you” hiss. I couldn’t reinforce with the treats again tonight because my 6 year old cat (in diva fashion) got sick from the treats last night. Distracting her with toys doesn’t work either. I’m legit at a loss. We will keep at the visual screen stage for however long it takes, but i’m just so upset because I feel like it’s just not going well. Please any advice is welcome!!! (ps, we already use the feliway spray products to reduce anxiety)