Preschool committee can get you like that.
Take a big breather and look at the issue of you being on the committee and the issue of your daughter attending as two separate things.
Is she happy there? Are you satisfied that she is well cared for and doing interesting, stimulating things? Are you happy that the preschool meets its responsibilities regarding health and safety, adult:child ratios and so on? If so, keep her there. If not, pull her out and go somewhere else.
As for you being on the committee, I can well and truly identify with how you're feeling. It is true that the committee collectively can be held responsible for various things, although the most high risk positions are Chair, Treasurer and sometimes Secretary. All this should really have been explained to you before you signed up, but if not, then you need to read up on it. Might be worth contacting your local Sure Start Partnership as they sometimes run training for new committee members.
It sounds like your Chair is a bit of a control freak. There's an argument to say that the best way to change something is from the inside, so if you have the energy and the motivation, then challenge her. Ask for an item to be put on the agenda for the next committee meeting to raise your concerns about things being agreed (and presumably minuted?) and then changed without the agreement of the committee. Or be more diplomatic and say 'Please give me some specific area of responsibility because I can see that you have a big workload and nobody helping you'.
However, you do it for free and out of goodwill. When I was a committee Chair I put in about 20 hours a week of hard work for nothing (and loved it). Now I'm a preschool business manager and I look at what our Chair and Treasurer are doing for free, and think they're crazy to take on all this extra stress and hassle for absolutely no gain. If you do decide to step down, put it in writing to the Chair. If you don't want to rock the boat by giving your real reasons, just say that due to 'personal reasons' you no longer feel able to make a commitment to the role and wish to step down. Make sure they notify Ofsted to get you taken off again.
Sorry for writing an essay but I hope some of it might be helpful.