Calcium - I've seen you on a number of threads, and like Mears, and I'm sure many others, I'm becoming concerned for you. I'm only a few months further into this parenting thing, and so have not the wealth of experience that many of the other women (and men) on here have, and so (contrary to what the number of posts might suggest) I'm reticent about giving advice - all I can say is that your situation sounds very similar to what I was experiencing 3 months ago, and let you know that things have entirely turned around in the last 3 months.
My ds was feeding v. often - he'd rival your dd - and would not accept a routine - I was shattered and at my witts end.
For me, the only way I could get through the day was to deal with it all as a job of work - I felt no joy in my new role as a mother and felt that I must be in some way inept as I couldn't get ds to do all those wonderful things you hear about, like sleep. I was due to return to work when ds was 18 weeks (decision taken in early pregnancy)and was dreading it - I could not conceive how I would ever manage, he wouldn't even drink milk (even ebm) from a bottle.
I eventually just gave in and went with the flow - the path of least resistance - taking him to bed to feed through the night if need be, giving up on any suggestion of a routine and that model of a perfect bab. We then found a way of getting on (ds & me), somehow I got used to the broken sleep and started to feel some connection with him other than obligation (before then I was starting to think that this bond thing was a cruel myth). ds eventually expanded his tummy enough that he could fill up to last longer than a couple of hours, and my energy levels increased further.
ds is now 6mths, sleeps upto8hrs and is a joy to behold, but is still ravenous - eating us out of wee glass pots, and drinking me & the chemist dry.
Please don't feel you have to manage this alone - use any help you can find, jadgole or blackmail - friends, family, professionals (if your willing to let us know your location I'm sure a couple of mumsnetters would be happy to meet you - I'm in SE Scotland) insist your dp is of more use, and most of all, please don't feel duty bound into meeting someone else's expectations.
If you wish to, and can afford to delay returning to work on the pre-ordained date, don't feel forced into complying with decisions made many months ago - that has been my one big regret of ds's early life. I feel that I was only just waking up to him when I returned to work, and it was too soon for me. On the other hand - time away may be what you're seeking, in which case go for it.
Keep in touch.