It's not either. It's "I am stressed out by the demands of my life right now and the grass looks greener." I have experience with caring for a disabled loved one. It's incredibly stressful and took a toll on my marriage as well. We survived. This is a lot to deal with in your mid 20's, but that's the life you were dealt. Sounds like you're the grown up here and ready to deal with it. She isn't (right now)
Your daughter is your main concern, not your wife's happiness right now. Sorry, but happiness is a state of mind, you little girl's disabilities are not. They are real and she's little and the best thing for her is to have two loving parents there for her. She's going to have enough of a tough road without a bunch of drama thrown in.
Get to a good therapist. One that specializes in FAMILY counseling, not just marriage. A specialty in the field will make all the difference. If, after 3-5 weeks you don't make head way, FIRE him/her and get another. A good one is hard to find. Tip: A therapist that just wants you to sit there and bitch about the other is not a good one. You want one that, after you give your backgrounds and identify the problem, will help you move FORWARD. Not just sit there note taking while you pay them to argue in front of them.
I wish you all the best. And if, after therapy your wife still is isn't on board with being a wife to you and a mother to your girl, THEN come back to this thread and let's talk.