Have you had your Free T3 and Free T4 thyroid levels checked?
Brain fog is linked to low T3 hormone.
Have you been on any medication for your underactive thyroid in the whole 35 years? It could well be, if you are on some, that they need adjusting. Often as we age, lose weight, or become pregnant, the levels needs a tweek.
Personally I am in a bad hypo state at present and I force myself to do brain excersises. They help enourmously! Maybe try some. I recite the Army Alphabet, a for alpha, b for bravo, c for charlie etc. Got to keep those neurons connecting.
Perhaps have a print out of the routine you are doing and if you do get flustered just glance at it and I am sure you will jog your memory.
Go to a neurologist and ask to have a neuropsych test done. it helps them determine what is causing the memory issues. All they do is a bunch of test and memory test with you to help you find out whats wrong. Good Luck
I know how frustrating that can be. I am hypo-thyroidal, I have Sjogren's Syndrome and Mixed Connective Tissue Disease, all of which can cause brain fog. I have to make a lot of lists. Carry a little notepad and pen. I have found this helps me quite a bit. Conversations still suffer, but for your classes a simple outline and a couple of key words to jog your memory may help a lot.