Oh hun you'll do fine!! DD2 was posterior (although nobody workd that out until her head popped out and the midwife went, 'Oh, well would you look at that? No wonder you had trouble pushing!') and I didn't find it that much worse than DD1. I only had a couple of puffs on the gas and in all honesty, labouring flat on my back with my knees up was the only position I could push in - for some reason I felt like I couldn't bear down properly (the pushing sensation was all in my stomach and not down into my hips the way it's meant to feel) and being flat on my back seemed to only way to give me the 'leverage' I needed to get her out.
It was a short labour, maybe an hour and a half to two hours of pushing, and like I said I didn't need drugs or intervention of any kind. Just try to relax, know that your back is going to hurt like all-get-out unless baby decides to turn around, but that you can do it!! Good luck