My first son was posterior and I had terrible backache during labour. That said I made it to 9cm dilated before I got to hospital (on no drugs other than endorphins LOL) and asked for gas once I arrive but was told it wouldn't do anything (which it didn't).
I ended up pushing out DS on my BACK and my legs up (yes, you read correctly - it was my OB's bright idea that I do that!) without forceps or ventouse. Pushing took about 2-3 hours. I had an episotomy but otherwise no ill efefcts - I was doing yoga two days after he was born

So having a posterior baby does not necessarily mean c-section, epidural or forceps. It can mean unnecessary pain (if you are told to lie ON YOUR BACK!) and a long pushing stage.

My suggestion is read up on alternative positions, get your birth partner to word-up the midwives (if need be) and make sure no one puts you on your back. This last part is hard to do yourself because labouring women don't exactly tend to have their full wits about them lol so see if someone can do this for you.

You can do it. Posterior baby does not equal drugs or intervention!