Ugh
Dangly I get so cross when I see things like this, in the last year or so I have seen so many similar stories 
You already have your diagnosis, one that means you should be getting life long injection & your new doctor is ignorant & very negligent for not continuing your treatment. The supplements wont work for you I'm afraid as you cant absorb it via the gut as normal people can as a meat eater, you couldn't be deficient if you could absorb it via your gut, if the supplements are under the tongue ones, you might be able to absorb a small amount of it if you hold the pills there a long time, but it will never be enough to,keep you well.
NICE guidelines & others that dictate hey are not meant to retest B12 AFTER B12 injections, other than to make sure its working, as of course your results will be fine, you've had treatment, it won't stay fine though as you clearly have a problem absorbing B12, which somehow affects the livers ability to store it (Im not sure of the mechanics of that, but often see it discussed on a B12 group I'm a member of)
Folate deficiency anaemia I've often read has the same symptoms of B12 too, they work together along with iron, magnesium & pottasium, so if one is out of whack, it will have a knock on effect. There is another genetic version of B12/Folate deficiency which is even more complicated than Pernicious Anaemia & I wont pretend I understand it yet, but if you have this MTHFR gene, you have problems with absorbing folate too.
Currently looking into that for my daughter, as she seems to heal better with folate supplements alongside her B12 jabs, instead of the cheaper Folic Acid, which a normal body should convert to Folate - we are waiting on the results of genetic testing to see if she has this gene, though our GP couldn't test for it, (its not on her list with DDs diagnosis) so we have used 23andME for private testing as it needs ruling out as its more complicated to treat, plus theres apparently a link to EDS, which we also have.
THIS LINK
has a ton of good info all about B12 deficiency & how you should be treat, plus on the "useful links" page, follow the link to "Pats Kormics Facebook Group" that group has been a godsend to us & has given me the info & support needed to fight our own doctors & make them follow NICE guidelines to treat both myself & DD. My DD was very ill, she could barely manage a wheelchair, let alone walk & has been to unwell for school for over 18 months - the effect of the B12 injections taken with co factors was nothing short of miraculous. I highly recommend joining the group & asking for help on there - they can also point you to a good MTFTH group too