I could have written your post when I had my 1st....he is now 10 and I have 3! I missed my old life too. I felt like I started too late also and that was why it was so difficult. I see teen moms and just wonder how they do it and I always assume it's because that's all they know(my sister).
A high needs child is so hard, draining along with all the good too. We too wondered what we had done wrong. I got so resentful of friends babies.
You are over a huge hurdle, you really are. Slowly you will get your life back, or a new version of it. No one could do what you just have with a high needs baby always, if it never changed and this was it. Like others said, she will change so fast. You gave her the best and it's not lost on her. My son is a wonderful caring boy now. You would never know he was such a high need baby. He is calm, quiet even and just really mild mannered.
Just keep doing the great job you and your husband are and stay strong. I know it seems never ending but it will pass.