I'll caveat this with the fact that my child's "disability" is very mild. My oldest son (15) has ADHD and learning disabilities. School is hell for him, but other than that, he has been able to make good friends and find activities (guitar, hockey) that are a good fit for him. I realize that compared with children who have developmental delays and physical impairments that this is small potatoes, but I wanted to let you know that even diagnoses as mild as his gave me pause and made me mourn a bit for what I saw as lost potential. I was an excellent student who loved, loved, loved school and found school to be the most positive, validating experience in the world. To know that he would never know that feeling was hard to take. To know that his efforts would be disproportionate to his rewards made me sad for him. My middle son (9) is also "off" somehow and we don't quite know what's up yet. He's very smart but very disruptive and obnoxious, possibly ADHD. Whatever the case turns out to be, he's another one who will need special attention and handling along the way.
Two things have helped me. First, having a child who is not typical has made me a much better person. My oldest son has stretched me and challenged me from day one. I am a better mom, better friend, better educator, better advocate, better person because he has challenged me to be so. That's his gift to me. To make me compassionate, and aware. To make me use the things that come easily to me (learning new info, writing, speaking, participating in things) to advocate for those who struggle. I do test prep instruction on the side and without living through this with my son, I wouldn't be able to help my LD students and their parents through the test prep process and help them to succeed as well as I can, because I do see it, and live it, and get it. I wouldn't want to go back to the somewhat oblivious (and arrogant) person I was before, who thought that everyone shared the "if you try hard enough, you'll succeed" experience when we know that that's just not true.
The second thing that helps me is perspective. I know someone whose little boy has mitochondrial disease. He just started Kindergarten this year, a milestone no one thought he'd reach. He treasures every day of getting to go to school and hang out with his new friends. Off he goes each day with his oxygen tank in his back pack, his gastro-stimulator "button" in his belly, ready to take on a new day and love it, even though it wears out his body and leaves him exhausted for the rest of the day. His parents and siblings have no idea how long they'll have with him (they know many other mito kids who have died younger than their son, or by age 12 or 13). My kids are all fine in comparison.
I have another friend who has a disabled adult son and whose youngest son, now 5, has quadriplegic cerebral palsy. He can't move anything below his neck. He can't speak at all. He can't eat or breathe on his own. He's legally blind. He lives in a hospital bed in his family room, hooked up to a ventilator, a g-tube and a j-tube. But he recognizes his family, and he smiles, and he hears, and makes noises to music. Again, the cross my kids and I bear is non-existent next to the one borne by this boy and his family.
I think it's great and healthy that you recognize your feelings and are talking to someone about them. I think you need to give yourself permission to grieve and mourn so that you can then focus on the positive and keep looking forward. And try to think about how being your daugther's mom has changed you for the better, and keep perspective.