Unisom is contraindicated while breastfeeding, at least according to an older edition of Hale's that I searched on Amazon. I took it for about a week for morning sickness while nursing my 2 year old (not an infant) and it didn't effect her adversely, but it did seem to impact my supply. (I don't know for sure because pregnancy is wreaking its own havoc on milk production.)
I've asked my husband who is a pediatrician as well as the family practice doctor that my daughter and I both see and neither of them can tell me for sure WHY Unisom is contraindicated. My husband's best guess is that it's either because a) it could cause apnea in a baby (but probably not a toddler) and/or b) it could have the same negative effect on milk supply as Sudafed.
I know that the OP said it doesn't work for her, but Benadryl is safe while nursing and isn't supposed to dry up your milk. (I've taken it many times over the past 2.5 years because it's one of the few allergy meds that is ok.)
It really isn't safe though to take any over the counter sleep aid while co-sleeping with a baby.
My daughter was also an all-night nurser and I remember how difficult interrupted sleep was to deal with. Do you have a partner who can help with night duty--even if just for a few nights--so you can get a little more sleep? Or would someone be able to watch her for a solid chunk of time during the day so you could sleep? At 6 months your breastfeeding relationship should be well-established enough that it's probably ok to replace one or two feedings with expressed breast milk in a cup or bottle. Are you able to nurse while lying down? My daughter didn't night wean until 29 months, but night nursing got A LOT easier once I mastered nursing while lying down and she figured out how to latch herself on without my help.