You are most definitely NOT alone!
It took 11 months for my son to get diagnosed, and those 11 months are a blur of no sleep, worry, research, dozens of dr appts, severe illnesses, surgeries...and that was just him. Also in that first year I changed jobs (got out of the military because of him and his needs), moved houses, my husband quit work to stay home with him, my oldest went through horrible behavior issues, etc. It was an incredibly stressful time.
Looking back, yes I had a version of post partum depression. Everything got easier for me once we had the diagnosis and I had something to focus my research/energies on. I had a "name" for this thing wrong with my baby. My husband hit his own depressive period in the second year, though. He was not prepared for the diagnosis, so what came as a relief to me came as a shockwave to him. It was a hard time in our family for sure.
I had a few friends that I clung to a lot, and I came here a lot to talk about the stresses. That helped a lot and got me through that time. My husband still struggles, he tends to struggle with depression even in the absence of stressors, so even though our son is 3.5 years old, my husband hasn't been able to pull himself out of the initial "bottom out". Doesn't help that we've since had another baby that has brought him own challenges (although he is thankfully healthy).
Hang in there, join us here, write it all out, every thought you have, we all understand and will be here to give you a virtual hug. Ask your daughter's specialists for recommendations to counselors, books, videos, webpages, support groups, etc. There is help out there!!