I really wish I knew the answer to this!
Bella suffers too as you describe. We stopped letting her go outside to see if it helped. Recently, let her outside to play because the weather was so nice & she dearly Loves it. Well, the last couple of days, she's biting, scratching like crazy. I'm calling the Vet again in the morning. She also has been on a low dosage of Temaril P in the past, but I did not want to continue it. The Vet wanted to try Ivermectin for mites. We may go that route. Benadryl does help. Will let you know how it goes. I'm so sorry your baby suffers with this. We know how miserable it is too!