I'm not going to be the best one to advise you, but I can at least offer some support and reassurance and commiseration.
I know how lonely it feels, but you're in the right place. There's so much good help here!

My best advice to you is to let go of the regret as soon as you can. If you're not dead, you're not done! I can say this because I have had many of the same feelings for many of the same reasons. My DS is 6.5 and we went through this exactly at the start of the year, when he was newly 6 and starting 1st grade. You DD will recover. They're resiliant at this age, and you are such a good mom for catching the problem so early! Please stop kicking yourself long enough to pat yourself on the back, okay?!

(It's okay if you continue kicking the school, however! Figuratively, at least.)
Plus, since you can't turn back the clock, it's best to get through that (natural) mourning period for what is lost as quickly as you can, so that you can get to the more productive phase of this process--a process that we ALL go through, BTW: taking action!
Lots of people afterschool, so there's lots of help available there. What is your daughter ready for in math and reading? What can she do now? Details will probably help. Would she do better with a workbook or a computer program or manipulatives like tangrams?
As for the advocacy: I guess my first question is what do you think should be done with/for/about her? What does your gut tell you? What do you want? What would your ideal educational situation look like for her?
Starting there is about the easiest place, I think. Then consider what is possible in your school system. Do they ever grade skip? How would you feel about that? Do they allow subject acceleration? Do they have a GT program, and if so when does it start, is it likely to be good enough for her to make it worth trying to get her in, etc.?
Then consider alternatives to your school system. Are there other schools available in the area that might be a better fit for her? How about homeschooling? (Harder to do as a single parent, but it can be done, especially if you can find good childcare help.)
Above all, keep your chin up. You've just taken the first step on a long and twisty path, full of lovely high peaks and dark, sad valleys. It's normal to mourn the simple education you thought your child would have once you realize that NOTHING is going to be easy! But the big, bright spot is that you're coming out of a valley now, and now you'll have some great companions along the way.
