You are definitely, definitely not doing her harm. You are sleeping and giving her comfort and food the way the human race was designed to receive it. The concept of 'approximately eight breastfeeds per 24 hrs' for a baby of your DD's age is, again, very Western. In traditional cultures, which mimic our prehistoric ancestors more closely than we do, babies are recorded as feeding an average of twenty times in 24 hours. Some of that has GOT to be through the night as well! So your DD's behaviour is normal and healthy, and you are supporting biologically normal, healthy baby sleeping and feeding patterns, not harming her AT ALL.
I think sleeping through the night is a holy grail, and you'll stay saner assuming it's not going to happen for at least the first year of your baby's life, then be pleasantly surprised if she does.
Is she really waking up more often? In his cot, my DS is awake every 90mins to 2hrs. In my bed, he's awake every 90 mins to 2hrs. The difference is when he's in bed with me, I rouse slightly for 10 minutes to feed him lying down, then knock off again, instead of waking fully for 30-45mins (or longer) making several abortive attempts to get him back in the cot which gets more difficult as the night progresses. More sleep for both of us.
WRT to thinking you might have PND, I can't encourage you enough to talk to your HV or GP. Antidepressants are very effective and the SSRIs (e.g. the really common ones like Prozac) are quite safe to take while continuing to BF. I sense that you can't see an end to this, which might be a barrier to being able to cope with the situation. Sometimes anti-depressants can genuinely help you to believe it when people say, 'she won't be like this forever'. Because she honestly won't 