Hey JustME,
I've been diagnosed with B12 deficiency about a year before I knew I had ME.
The first round of shots didn't do much for me. A couple of months later I was still showing a megaloblastic anemia which can be a sign of B12 deficiency (but also of folate and B6 if I remember correctly).
This time I received daily shots for a week, then 1 shot a week for 4 weeks and then one a month. That worked miracles for me. No more dizziness and brainfog and much improved strength and energy. That lasted for about 10 weeks before I declined again, and no amount of B12 was able to stop that.
I have since found out I am severely deficient in at least half a dozen other things that are essential for normal mitochondrial function and ATP pruduction. So my best guess is that the B12 worked well until the bottle neck was somewhere else, and some of these factors are impossible to replete for me, like carnitine or Q10.
I also would like to encourage you to try all different forms of B12, (adenosyl hasn't been named on this thread I think), they all do different things for me.
Many people have reported good improvement on various B12 regimes, so I hope you will benefit a lot from it, too!