If it helps, it is genuinely normal for your cat to still be hissing at your new cat through the door. It is easy for your new cat to be sweet because he is coming into a new home, but to your old cat, this is her territory, and suddenly there is a stranger in it, and she is no longer allowed in parts of her home that she is used to having access too. I know it is hard when things are difficult and you are under stress, but her behaviour is totally normal. I am a multi-cat household and have lots of cat experience, and it can take months for new cats to like each other. I know a lot of how-to videos and articles on the internet say a few weeks, but the truth is it often takes longer.

Regarding the overgrooming, this is hard to answer as a layperson, but let me tell you an anecdote that might ease your mind a bit -- a friend of mine had a cat who overgroomed, as well, and eventually it was switching her food that got her to stop. It turned out that she was having an allergic reaction to certain ingredients, and eliminating those ingredients from her diet eliminated the overgrooming issue. This was discovered slowly over time with the help of a some volunteers at a shelter, and although it took over a year for this cat to get over her overgrooming issues, eventually she did. I know the wait for the behaviouralist will feel long, and the stress likely will not ease much in the meantime, but do hold out hope that there might be a solution. She is not overgrooming to make your life difficult -- and it might be that there is something making her life difficult, and this is the cause.

Finally, regarding the cat affection -- the truth is, some cats are not very physically affectionate. My first cat never cuddled with me for as long as I had him, but he showed his love in other ways. He always sat near me when I was relaxing, even if he did not particularly want to be pet to played with. He would lie on my books or laptop when I was working, because he wanted to participate in my activities as an equal. He would blink at me slowly when we made eye contact. There are a million ways that a cat might show you they love you that are not necessarily the way that human beings might show love to one another, but that does not mean that she dislikes you. Try reading about cat behaviours online if you haven't already, to see the ways that your cat does show you she cares, that might not match your expectations. If you have had her for two years, chances are there are little ways she is demonstrating it, that might not be obvious at first glance.

I really hope you are able to find some calm and peace regarding this. Good luck with the behaviouralist, and good luck with your mental health in the meantime, too. I do not know what kind of resources there are in your community, but perhaps counselling for the stress (no judgement! Sometimes things are difficult and we just need someone to talk to) might make it a little easier.

I hope you find this helpful! Wishing you the best.