Hello Nuch:
The reason that your blood values of B-12 are normal even when the person is low is that the normal range is taken from a population that is notoriously low in B-12. They are "average diet Americans" walking into the phlebotomists to get blood drawn usually when they are sick. For B-12, the serum levels are not even the best way to measure it. Though the last time I heard, the correct measurement was still only done by one lab on the East coast.
That being said, I would say you are VERY low and supplementing should definitely be an option. In fact, I would go so far as to say that injectable B-12 would not hurt one bit. However, if you can not get your physician to agree to this, I would take a B-12 supplement called "B-12 Dots" it is by "Twin labs" and they are tiny pills that you put under your tongue and allow to dissolve, you put one under your tongue in the morning daily. There are also good liquid preparations on the market at reputable health food stores that are very helpful, most are mixed with Folic acid which is essential for the B-12 to work. Don't forget to add other B vitamins to your diet as well. They all work "synergetically" and you need a good "foundation" for optimum effect.
I hope this helps, if you have any other questions, feel free to ask.
Susie