I think you should read "Three in a Bed", not because I think you should co-sleep, but because it will help you see things from her perspective.
There is nothing natural or normal about babies sleeping for 12 hours without waking up, it is perfectly normal for babies and toddlers to wake once or twice in the night, when bf mothers co-sleep with thier babies/toddlers the child will nurse a few times in the night. When you remember that bf/co-sleeping is the natural way for humans then you realise that this "Holy Grail" of 12 hours sleep is just that, a myth, something that doesn't really exist!!
You say she isa clingy baby, so embrace this, meet her need for contact and cuddles and reassurance. Carry her in a sling (like a ring sling or an asian baby carrier) during the day, you might be amazed by how this changes her. She is just a baby, she needs food and comfort, and a well rested Mummy. So, ignore the CC type advice, stop seperating her (keep her in your room, or better still your bed!) and ride it out. It's not a "bad habit" which can be broken, it's a fairly natural phase which will pass. The key is to find a way to survive it, not try and stop it before it has run it's course. Think outside the box, ignore "convention" and do the things that work, even if it's a dream feed or co-sleeping or a 3 am feed. It's a short term thing, you won't be giving her a bottle at 3am when she's 10, so don't stress about it.
And before you say "thats easy for you to say", I haven't had a full nights sleep for nearly 5 years, if I'm lucky I get about 5 hours unbroken each night. I also work, and have an "I'm so tired/I work so hard" DP, so I do know exactly how you feel. For me the thing that stopped night-waking driving me crazy was to accept it as normal and stop thinking of it as a problem (or sign that there was something wrong, either with my child or my parenting!)
But don't leave your baby to cry in her cot in a room away from you. You are her mummy, the one thing in the world that she should be able to always rely on is that you will come when she cries for you.
[hug smiley] It will get better, I promise.
Monkies
(PS, sedation like Medised, Piriton etc won't stop the 3am waking as it would have worn off by 3am! Meds like this last for about 6 hours, so if you gave it to her at 7pm it would stop working at about 1am. Sorry!)