Dear Mo, "Brainfog" can be the result of toxins released into the bloodstream when cleansing, stress, allergies and even be a part of the progression of MS. The lady with MS I used to care for would have spells of brainfog where her memory was shot and she had trouble communicating. The next time I would see her, she would be sharp as a tack. Once again I suggest you drink a couple of glasses of carrot juice daily to help cleans and feed the body and help keep your blood sugar at a constant level without sudden spikes and dips.
I am a Christian, but I can relate to some of your feelings. I have always tried to keep a servant's heart and help people, even in small ways. It was so hard to have to give up things like Bible teaching when my health problems put me in a wheelchair. It is hard to be so poor when my heart wants to be generous and giving. It is difficult to accept help when I was often the one who tried to help others. Still, I have learned that others are blessed when they reach out and help me, I have been challenged to give even in times of financial difficulty, I miss teaching, but have found I still have much to learn. It is very difficult for me to feel "blessed" by my situation; but I do believe, in both our cases, that there is a reason and a purpose for what we are dealing with. Do not give up on hope, do not ever stop fighting to get well, but always be strong in the fact the God loves you and will not place more on you than you can bear. You may be the source of blessing to someone right now, perhaps a world away, but still getting strength from you. Accept the brain fog as a time you need to set aside for yourself. Turn inward in prayer and meditation. You will be blessed with opprtunities to help others, but you must also be gentle and kind with yourself and your needs.
By the way, I have nerve damage from a spinal injury. It is not MS, but I can relate to some of the things you must deal with. My doctor told me that as long as I have pain it means the nerves are not dead and there is still hope for regeneration. I find I can even be thankful for pain (although on days like this I would also be thankful to have some respite from pain too).
Keep your faith, but hang on to hope and open to suggestions that may be of help to you.