From what I understand, you are not saying you couldn't LOVE a disabled child, you are saying you fear you would not be up to the job of CARING for a disabled child.
I think that if you feel you cannot give a child, any child, everything they need, then you OWE it to that child to make sure you get them someone who can. Be that adoption, respite or home help or whatever.
I think though, that your fear is born of ignorance - not saying that in a put-you-down way, but if you have no experience or understanding of certain disabilities, I suppose it can be difficult to process for some people.
But the thing to remember is that children don't come with guarentees. You have no idea if the 2 children you already have, will one day need full time care - will you give them up for adoption if so? Or will you look at other choices?
Do you really fear that you are SO incapable of looking after your child if they needed so much care - from birth, or as a result of accident or illness later on, that you would feel you had to pass that to someone else without trying to get help, support, training first? If you really feel that, then I think you would be sensible to give them up if that happened, because you wouldn't be the best person for them.
I think that you will go round in circles now, but you have NO idea how you will feel if you are ever IN that situation.