OK, first of all let me assure you that there are anti d's that are safe to take in breastfeeding - One of our now retired mods needed them after the birth of her baby... You don't have to deal with it all alone.

I had pnd with each of my babies and it affected me in very different ways with each of them.
You aren't turning into your Mum. You know the signs, you know what to look out for and actually talking things through with your partner is the best option.