Hi there, I had the same worries. Really bad PND with DD, hospitalised with it as well. I found the sleep deprivation absolute torture, and failed to understand how anyone could revel in the newborn days. Like you, it probably took a full year to start enjoying being a mum and feel even slightly "normal" again. Even then, it still felt like a daily struggle.
DD is now 3.5 and I am just about to have DC2! (38 weeks pregnant!) Can't say it was an easy decision to reach, but we got there and firmly believe it was the right one (maybe ask me again in a month or so!!)
Basically I felt that although we'd had an horrific time first time round, there was a lot that could be done differently to help this time / prevent it happening again. And I didn't want DD to be an only child. I'm being much more closely monitored this time by mental health services, and we've got support in place. I think for me, having a decent age gap of 3.5 years is a good thing - DD will be starting school nursery, I will have a bit of time each day to myself (well, myself and the baby) and she is that bit older so more independent / able to explain things to / able to fetch things / be helpful etc. I don't think I would have been ready for number 2, had DD been any younger than she is now TBH.
It's such a personal decision. What does your DH / DP think? He needs to be fully on board to support you after the birth. And your HV needs to be well aware too so support can be put in place to make sure you don't get so low again. Were you on medication>? You may need to get it checked out if you plan to get pregnant on ADs. I managed to get off all my medication, but it wasn't easy, that is a whole another story!
Although I feel a bit anxious, I do actually feel really positive now. I came through PND. I love DD now and I love being a mummy. PND was not the end of my life, I survived it, and if necessary I will survive it again.