Well done for posting on the forum, it can be really hard to open up as a mother and it's brave to come on here and talk about it, I think it's something that more mums should do.
Firstly, I just want to say that what you are going through is really tough, being a mum can be the most isolating thing in the world, especially without the help of your partner or family. I think it is so reasonable that you are struggling right now. I definitely found being pregnant with a toddler one of the most difficult times of my entire life, I didn't know what to do with each day as I was so tired but my toddler needed soooo much attention! You must be so exhausted.
It's great that you can recognise that you are in a place that you don't want to be right now and have made an appointment with your doctor, did you have a counsellor you liked form when you had PND? Or were there things that you did that helped you then?
I know this wont fix your current detached feelings, but what I found really helped was trying to find something (anything) to take my toddler to every day of the week. I know this might sound like a big ask, but for me getting him out of the house everyday made him much more manageable at home. I hooked up with the library, the playgroups, the music groups, swimming lessons... what ever I could find that was reasonably priced. It also gave him play opportunities that I couldn't give him at home because I was to exhausted to do/think of anything. Also I'm not sure if he's already in daycare or if you even want to consider it but having that break 1-2 days a week (especially if you're on 24/7) can be a lifesaver.
If you need someone to talk to I'm sure you've heard of PANDA, they specialise in PND, pregnancy and having young kids 1300 726 306 (open 9am-7.30pm) and there's beyondblue on 1300 22 4636 which is 24/7.
Feel free to jump back on the forum for a chat or to let us know how you're going.