In four months, she will not need the formula. Maybe if you stick it out for another four months, you will feel better about never giving her formula. Its really not that long of a time if you think about it :) Then, if she needs it, your husband can give her regular milk when they are out together. Besides, pretty soon she will be eating more foods and probably not nursing as much.
I understand the lack of sleep! My breastfed babies do not sleep well at all. My youngest, who turned 1 in June, is still nursing and still doesn't always sleep through the night. It can be hard, but I know it won't last forever. Im sure you have plenty of milk, perhaps its just not very fatty? My babies seem to nurse for short periods of time, but often. I dont think they get much of the fatty "hind" milk. Our pediatrician also told me that its common for moms milk to contain less calories (fat), after having multiple babies. If she is eating solids, try and feed her as close to bedtime as possible, so her tummy is full. Maybe someone else has more advice.....
As far as sex drive, mine is low too...but I'm thinking its because of the birth control I'm on.