I echo much of what LFL has said to you. Your W is having what I'd call a "Pavlovian" response to sex, and because you enable her to do this....she continues to do it because it works for her. If nothing else I think she may have a great deal of anxiety attached to "sex" in her mind....and therefore her body takes on it's own type of anxiety attack and she gets herself worked up to where she becomes ill. That my friend....is a BIG problem. Not one that can't be dealt with though, but you do have to find out what the root cause of her anxiety really is.
She doesn't have an opportunity to have this pavlovian response when you surprise her and don't do the build-up like you are talking about with V-Day. By doing this build-up you are giving her (or her mind/body) plenty of time to prepare a defense against you.....is that helpful? Nope. Is that productive for fixing your R? Nope. Are you enabling the behavior? Yep.
I know you enjoy doing these romantic things and using your creativity to do these things and that's wonderful. But wouldn't it be even better to be able to use these talents on a woman who will appreciate it like she used to? In order to turn things around in your R, you are going to have to begin changing your behavior....stop enabling her behavior. Stop letting that work for her.
Now, does that mean you ravage her when she really doesn't feel well, of course not. But I'm thinking there's some serious communication that needs to start happening between you two, and you are going to have to be the one to start it. It needs to be honest, blunt, and heart-felt. You might even want to consider counseling in order to help you both through this.
