Your feelings are perfectly natural, we all worry each time a cat needs some sort of surgical procedure,but no need to worry its a month away and if you worry from now until then you wont be well enough to nurse her better when the day comes,so relax or she will sense the tension and she needs to be stress free before having it done,
i know its not easy she is your kitten who is growing up,and your doing it for her long term health she is at an ideal age to be done,so before any anaesthetic is given to her the vet will check her over, such as heart beat and breathing and so on to be sure she is fit enough,so no need to worry but of course it just comes natural to us,
and while she is under the anaesthetic she will be monitored at all times, to be sure her breathing is stable and she has a good heartbeat and so on,if any problems they can reverse it quickly and bring her around, but complications are so rare its not worth worrying about it as they can wake her up safely,
and then she will be put in recovery, then you will get the call to say she is out of surgery and you can collect her that's the best feeling ever,but when you do get her she will be groggy don't be alarmed though it can be scary, i cried when i first saw a cat all drowsy,but within hours she was all alert my tears quickly dried as i tried to stop her climbing!
so my cats only had a small incision on the side and 3 stitches,the vet may give her a collar to wear to stop her biting at them,and they will give you an advice sheet on how to care for her,but its recommened that we only give light snacks, as with her being groggy she could vomit if she eats or drinks to much to soon,as being groggy she couldnt cough if she choked on her sick,
so for 24 hours its just light snack and a little to drink,then its watching in case she snags her stitches on things,and be carefull in case she tries to climb at first as she wont be able to judge very well with being drowsy,so if she has a climning frame hide it away for 24 hours as you dont want her tumbling,
so in 24 hours they are back to normal even if a bit sore,but after a week its as if they havent been for surgery,they heal so quickly its amazing just keep an eye on the wound for any discharge ect,and afterwards they are the same many worry about this but my cats were just as loving as ever,only difference was they were more homely as they were not interested in male cats,
so there you have it,you really dont have anything to worry about at all,i used to foster many cats so saw a lot being fixed, all sailed through it without problems,i though was a nervous with wreck trying to keep them still while they recovered!
hope i have put your mind at rest.
Always had british short hair cats,all fixed no problems, am in the UK.