The saddest part for me is that you think that you are horrified by the idea of your child being autistic.
Do you know any autistic adults? Do you know any autistic children?
It's such a vast array of characteristics, and no two people have the same combination to the same extent, at the same time.
If your child is autistic, you will deal with it. You will find so much that is somewhere between ok, to good, to amazing.
I am relieved I didn't know the signs when my son was this age, as I know I would've been so worried. Unlike the others, I think from what you say, this may be something that is eventually diagnosed, but from what you're saying your absolutely cannot say her will be non verbal, or anything else.
My son didn't walk until 18 months. Never crawled. Can't remember when he started talking but it was late. Couldn't read when he went to school.
He is the most articulate young adult I've ever met. He is extremely bright, and at 9 had a reading age off the charts. So do I wish he didn't have to deal with some of the things he does? Of course. I would say, though, in our case, there's nothing wrong with him when he's in familiar territory, but finds the world hard work and sometimes inexplicable. Yes, you have to mourn some expectations, and what YOU want for a child of yours. But that's your issue, not theirs. When it gets hard it's sometimes helpful to look at the person. Because you will see a whole person, not a set of atypical character traits. If it gets too tough, it's usually because I'm trying to make something work to my preconceived idea about something. If I really listen, really observe, I usually see that there is a way through that day's problem, if only because, with time, most things change/pass.
Please do get the help, get different medicine. Just because one isn't right, doesn't mean another won't be. There is one that's relatively unusual that is amazing - I've seen it used for OCD, and meanwhile the catastrophising and deep anxiety really lifted. I think it was clomipramine? The only time I heard of someone having something that wasn't an SSRI.
Anyway, therapy, medicine, reframing your fright, will all help. Remember, when you're walking through the reality of your life one day at a time, it is never awful every day. I've definitely had some of my darkest days over this very thing, but have dealt with far more difficult issues.
Whether your son is autistic or not, it will gradually become your new normal, and what worries me is that as your concerns are replaced with love, and skills, and just plain knowledge, something ELSE will take its place in running your mental health, and ruining your life. It doesn't sound as if having an autistic child would ruin your life, so much as the fear of the unknown.
Good luck.