My son has a male friend who cannot build muscle and remains weak.
No amount of working out works for him(he also has lowT and is in his twenties like you.

After much evaluation by a doctor, this doctor decided to test him with a DNA test for Klinefelter's syndrome. (fragileX).
These men have 2 X chromosomes and 1 Y. They often go undetected for life, unless some medical reason is present for further testing. As expected, my son's friend tested positive for this. Klinefelter patients have many signs of ADHD, poor energy, and slowed cognition.

Here is a more detailed article:
http://ghr.nlm.nih.gov/condition/klinefelter-syndrome
Not all Klinefelter patients demonstrate all the physical characteristics. But lowT is the most common indicator.

Since you are exploring so many avenues for your symptoms, I'd get that DNA evaluation to rule out Klinfelter's. LowT in a young male can also be due to some pituitary dysfunctions, and an endocrinologist is best to see for that aspect.
You will need to take that B12 with an empty stomach, and keep it in the frig once opened. MethylB12 is not stable in heat, in liquid form.

"Boosts" from B12 are not really present except in the first few doses. Was this before any supplement? You have to stop the supplement 3-7 days before having testing done, or false elevations will result.