Special diets will not work if she is allergic to fleas.
The best thing to do is to stop the cycle. If she is at the point that her skin is raw, she could have caused herself an infection or a yeast infection. If this is the case, you will have to get medication from the vet to heal her.
You are on the right plan by using the topical medications. Collars etc are a waste of money. And I would call the vet and ask if you can apply the treatment more frequently.
It can only take one lousy flea to set off the allergic response in the poor thing. Make sure when you are cleaning your home, you empty you vacuum outside as soon as you are done, otherwise the damn things just crawl out of the vacuum.
Btw, there is someone on here named 'DVMMOM'. Send her a private message. :)