Hi Emily, welcome to Mayo Connect. I am very sorry to hear about you having so many dire health issues. It's especially distressing when a person as young as you are is hit with health issues at a time in your life where you are still developing in many ways and have so much life ahead of you.
I firstly would suggest you start following the group on Connect which is related to GI issues, which would be here: https://connect.mayoclinic.org/group/digestive-gastrointestinal-problems/
In any case, what you have sounds pretty mysterious. I can understand your having anxiety and stress over this. What is your support system like? Do you live with your parents? Are they a good source of support for you? Other family? Is there any history in your family of illness, particularly anything related to the maladies you are suffering from? You have certainly run the gamut of medical practitioners. They don't (so far) seem to have been very helpful to you ultimately. After all the people who have seen you, Nexium is where you have arrived at? At this point you sound like a candidate for Mayo Clinic or a research related hospital. Have you considered this? The very restrictive diet and continuing distress from abdominal pain make going to college problematic until you at least get a clear diagnosis and treatment plan. Just my opinion. FYI: the Mayo appointment webpage is easily found from mayoclinic.org.
I wish you well, I hope others here can offer some guidance as well, and I would like to know more about the support in your life as well as your plans for seeking further help and more effective treatment. Best to you Emily, Hank