I'm so sorry to hear that you're experiencing the same. The week before Christmas we were finally forced to switch our daughter over to Neocate. There was nothing that I could safely eat anymore, and, since I have Crohn's, I was having a lot of trouble of my own, weight-wise, on such a restricted diet. I also had the opposite supply problem - not enough.
She is doing very well now in every way, but it was a harrowing week for everyone trying to rapidly wean her. I had a lot of grief, guilt, and, obviously, stress, but I'm feeling better about it emotionally now. I'm sorry I couldn't feed my child in the way that should be best, but I'm glad we at least have access to special formula for her needs, and that she can now thrive. Still a little sad, but it's good that we're both healthy and sleeping these days.
We saw an allergist, too, who initially gave me an attitude and clearly thought I was overreacting as well. (When one of our prick tests came back positive - for fish - she changed her tune.) It's terrible when you can't get the support or even information you need from the healthcare community. What was most helpful for me was searching out folks online who'd been through similar challenges - you might want to look at pediatric allergy sites with forums - it was actually far, far more helpful than any doctor or lactation consultant visit we made, because most just don't see this problem or this severity of this problem.
My daughter is 6 months now, but we'll be waiting until 9 months, on the advice of many parents, to introduce any solids at all. Another piece of advice I got was not to introduce grains first (like rice cereal,) but to start opposite of what you'd normally introduce - with a meat like turkey, and to stick with only one food before introducing another for a few days at a time, to measure reactions. I think we will probably start with sweet potatoes, bananas, or pears, since I'm doubtful she'll take to meat first.
There are of course many factors in a dilemma like this. If your little one is in good enough shape, gaining, mostly healthy, and you feel you can stick to such a restricted diet, (i.e., you don't have underlying health problems of your own, as I do) you could go on nursing, and shouldn't feel pressured to do otherwise. But I am glad now that we made the switch. It's wonderful not to have to worry constantly, it's wonderful that she's so much happier and well rested, it's wonderful that I feel I can take care of my child instead of being confined to my bed and the bathroom in every spare moment. But again, I don't know the ins and outs of your scenario. I wish you the best, however, and feel free to send me a message if you have any questions.