I had the same situation the few days before, luckily, I success to do a complete circle today, I think it is onkly about practice. (I practiced it for 4days before i can do a complete circle… :-()
Training Wheels my Brothers, Training Wheels. 
Actaually Riding in Backwards circles is killen me. Makes me almost angry on the bike. alwasy get 3/4. Maybe oneday il get it.
In the meanwhile. At least i can ride forward, It does seem a lot more practical.
you guys will eventually get it, try doing it out of toe clips/straps if you are a bit afraid of going down.
but once you crack that first full circle, you will get the feeling for it and getting through the point where you dont have the pedals/your feet to help balance you.
when I started, I leaned against a pole and push myself outbackwards then start from there…
Hey i find that concentrating on your front wheel and keeping a steady angle works, once you can get past the 3/4 mark your sweet for another rotation, but its pretty much look like a dick for a week untill you get it, then look like a hipster forever. 
you need to be able to ride a backwards circle in both directions, then if you trying to go ‘fakie’ nose bonk off a wall and crank hard out of it, much easier to to that then start from a track stand.