I have a larger age gap between my first two (27m) but just wanted to reassure you that you won't necessarily get PND again this time. I had it after no1, but not after no2. Things that helped me were:
Setting up a support structure while I was pregnant - getting my HV on board (she was fantastic); being in touch with the Peri-Natal Unit that helped me previously, so that they began visiting me as soon as dd was born, rather than waiting for me to get ill; having some therapy while I was pregnant, so that I could talk it out and address both the issues that had caused the PND and my fears about the future.
Filling the freezer with food. Whenever we cooked a dinner we would cook too much and feeze portions in foil takeaway containers, so that afterwards all I had to do was shove one or two in the oven for ds1 and myself, or even for dh as well if we didn't want to cook.
Talking with dh so that he could address his worries too. Being a 'strong male' he had never let on until then that he worried whether he was in any way responsible for the PND, and that he should have or could have done somethig to make it all better. He had to understand that this was not the case, and that he had always been my rock throughout. He also had to be given permission to find it hard going himself.
Allowing myself to cry, to be miserable, to be angry, to find it tough - and not to feel guilty about this. Having realistic expectations of myself - I didn't need to be supermum. When you've got a newborn and an older child, just having washed and dressed all 3 of you is an achievement - it doesn't matter at what time of the day you achieved it!
Knowing that a child doesn't need to be entertained for every waking hour, and that you don't need to put Cbeebies on all the time either, but that using Cbeebies is fine as well.
HTH