I read about your post on Autostraddle (a popular lesbian website that's been a great support system for me) and I'm usually not the type to post on forums, but I was moved by your post so I joined this website to give you a bit of encouragement, since I can relate to your and your daughter's circumstances.
As a college student from a small, conservative Southern town and a very conservative, religious family who has recently come to terms with my own sexuality, I can relate to your daughter. Even though I've always been very liberal politically and very supportive of LGBTQ equality, I grew up in an area that isn't -- for example, my high school banned gay couples from prom, and LGBTQ people in my state can be fired on the basis of their sexual orientation. When I went away to college and began to put together the pieces of the puzzle to fully grasp that I was gay, I felt horrified; I was afraid that I was going to go to Hell, and prayed that somehow I'd be turned straight. I was also terrified that I would have to hide my identity from my friends and family. It sent me into a deep depression and I struggled with it for a year before finally coming to realize that a) God still loves me, b) my friends still love me, and c) my mother and my sister will eventually come to terms with this when I come out to them, and likely my Dad, too, although it may take him a long time. Please know: I fully support everyone in the LGBTQ community, but it's one thing to support others, and another thing entirely to see this in the mirror everyday. Coming out to myself was the hardest thing I've ever done.
I haven't come out to my family yet because we've been going through a tough time with my mother's and my sister's health, and I don't want to burden them (and also because my sister will tell the world, and I'm a very private person and don't want many others to know just yet). But mainly, I haven't come out because I couldn't ask others to accept this in me before I fully accepted it in myself, which I now have. I recognize that I will be a much better person for being gay: I more compassionate and open-minded, and feel that I've been put on this earth at this time, in this place, to make a difference and stand up for equality.
My personal recommendation would be to make LGBTQ-friendly remarks to your daughter (perhaps about a news story about gay rights) to let her know that you are open-minded (once you come to terms with this, in your own time). And reaffirm every chance you get that you love her and support her no matter what, because if she is coming to terms with this herself, this could very well be a difficult time for her. Be kind to yourself; understand that this process is not easy, especially for those living in conservative rural areas. And use PFLAG as a resource if you wish; I was hesitant to go because I was afraid that I'd see people I know there, who'd "out" me in the community, but PFLAG is supposed to be an anonymous "safe space." That being said, my local PFLAG is tiny and while it's great support for many LGBTQ people and their, it didn't really help me personally. What helped me most was finding support on the Internet, and seeing that there are so many other people out there who feel just as I do, and who go on to live happy, normal lives in which their sexuality isn't a big deal.
I'm sorry that this is a long post, and I don't mean to be egocentric by posting about my struggles; I just hope that perhaps it can help you and any other moms who are looking at this forum, who perhaps are wondering whether their children are LGBTQ. I understand how difficult this struggle is for you, and send you a hug and wish you all the best. In fact, big thanks and hugs to everyone who has posted -- you guys are awesome!!