Nothing is more frightening and discouraging that not having answers and going from one test to the next. I have gone through that a few times in my life and my family was a worried mess. Right now I have no idea if I have is Lupus, RA, or something else (Long story). It has been very frustrating.
Do not be hard on yourself about the weight gain. You are going through a rough spell and going from normal speed to almost stop can be very challenging emotionally and physically. It has been very hard on me to gain weight in the last few months, but I can't beat myself up and neither should you.
Also, has your neurologist evaluated you for MS? It is normal to have lesions on the MRI scan. I'm not a doctor but one time feared I might have MS and did some research about it. It can also cause great fatigue, decreased memory, dizziness, vision problems, etc. Like RA and Lupus, it can take years to diagnose because the symptoms are slow to develop. Also, there are treatments for it and these other illnesses so, even though it is challenging to deal with, there is hope for feeling better.
Whatever it is, do not forget that you are not alone. Other people have been where you are and felt what you feel and this board is a great support network. Regardless of what is determined, you will get through this. Your hubby will understand, too.
Please keep us posted on how your tests go. I hope you get some answers soon!