I would check out the Single Mamas forum for lots of great advice from women who have BTDT. Also, I would go to the library and go on-line and read up on divorce, your rights, how to make it work for your kids, etc.
As far as this attorney who hasn't called you back - forget about him/her. If they don't call you back now, that is NOT a good sign. Find someone else. Start calling all the ones listed in the book - you should get a free consultation from someone who will estimate up front how many hours your case will take, how much it will cost per hour, etc. You also need to find someone you trust and click with. Ask around - do you have any friends, relatives, or acquaintances in the area who have recommendations for divorce lawyers?
Do NOT make any final decisions tomorrow with the mediator. Although there is probably a strong impulse to get on with things, you need to know your rights and responsibilities and you need professional advice on alimony, child support, visitation rights, etc. In short, you need professional help - most especially because you have a special needs child, and your past and future care of her must be taken into consideration in any divorce settlement.
Good luck, mama. This will probably be a hard row to hoe, but from reading your other posts it sounds like a step in the right direction for you.