I haven’t taken b-12, but my mom took it for pernicious anemia, an autoimmune disorder that interferes with absorption. Have you been tested to find out if you are deficient in B12 or if you’re anemic? Is your appetite ok so that you’re eating enough? Getting good sleep? Have you discussed the fatigue with your doc?

I’m sorry you’re dealing with it-persistent fatigue can be depressing. I struggled with fatigue October-mid Feb due to a couple viruses and then another condition flared. Dragging myself out to exercise helped a bit, but my body having time to recover from the infections and getting the flare under control helped the most.
It’s good that you are exercising. I hope you get help and start feeling better soon..