My W also feels like a disappointment to me. She's even said that if she had known she had all of these sex problems, she never would have married.
The fact is, she HAS disappointed me, but I don't ever want to say that to her. She already has a poor enough self image and doesn't need me to put any additional dents in it. Like you, I want to assure her that she's not disappointment to me, even though, like you, I feel that she is.