Not your fault. And not wrong position. Not the "average" or "most common" position, but still one from which a non-intervened vaginal birth is perfectly perfectly possible (and your chances of that increase with every jot of confidence you can muster in your body's ability to DO THIS between now and labour day)
I had a loooong posterior labour. Gas and air (eurgh. I hallucinated. I must be the only woman in England who has had a bad trip on gas and air) for less than 2 hours of the labour. No other painkilling drugs. Some women like heat on their backs in this kind of labour. I had a bag of frozen peas. Lots of hefty back massage also a must.
If still back to back when you go into labour:
relax as much as you can, sleep as much as you can between contractions. Don't get yourself emotionally committed to labour until you're really roaring through contractions - it's going to be a long haul and the thing you most need is rest.
stay upright as much as you can (I did, even when I was taken into hospital and put on a drip to speed things along, and it really helped)
Birth on hands and knees/in a squat/something similarly upright. If possible, don't lie down when you are pushing - you need gravity on your side!
Be ready for the back pain. For many, it doesn't go away between contractions. I felt no contractions across my belly at all - the entire labour seemed to be happening in my lower back.
If your waters go, it may well just be a little bit of the waters, and if things are very very slow, you may want to invite the midwives to rupture the rest of the waters to bring the pressure of the head down onto the cervix. Or not. depends how long you've been at it. (Mine were artifically ruptured in the end and I gave birth within something like 2.5 hours from that point, at depressing cm dilated after hours of labour) It certainly makes things more intense...
I don't want to sound preachy - I just wish someone had given me a list like this last time! Posterior labour is ok, it's just a completely different animal from an anterior labour. And I guess both are quite quite different from the various kinds of breech.