(a) I simply can't sleep properly with her in our bed and (b) that would open up a whole world of trouble once (if) there is a baby in our bedroom waking up every few hours to feed...
beans The trouble is, you aren't getting any sleep anyway! I did it so that I could get some sleep. If your DD can come into your bed and feel welcome it might help to relax and reassure her. If you can, and have the space, why not invest in a big 6' bed - especially if you are planning to have another. Could your DH go into another room? Not permanently, obviously, but mine had to occasionally if he had an early morning/lots of driving/long day ahead.
It seemed awful at the time but we were both at the end of our tether and I suspect a lot more people do this than ever let on. I found the whole "mine sleeps through the night" thing competitive and tedious, and I've met mums who had an easy time with their first child and think they'd cracked it, only to have an ongoing sleepless nightmare with their second, wondering why nothing seemed to work.
You have to bear in mind that children are as individual as adults are, and some will sleep better than others - no matter what you do! DD is now 11 and is still a light sleeper, dreams a lot, wakes often, needs a dim nightlight - but no longer bothers us unless she's ill.
Yes it was terrible sometimes, and I often felt like the walking dead. Thankfully those days are over, and now they're over, you do look back and realise that in terms of your own life span, it's only a short time.
I really feel for you though!