Hi!
I have just had ds 3 (last Monday so 6 days old) and had worries just as you do. I had severe pnd with ds 1 and saw a psychologist for over a year. Ds 3 was planned and very wanted but I did/do have moments when I thought -what the heck have we done? We were on an even keel and really enjoying family life.
My boys ages are ds 1 -5 and ds 2 -20 months (and also a non-walker at the moment)
What we did to help;
-After delivery with ds 2 I had a pessary to regulate my hormones called cyclogest. It was prescribed for 3 days and is designed to help prevent the dip on the 3rd day. I have had it again with ds 3 for 5 days. My psychologist recommended this.
-Both my elder ds's were around to meet their brother and spent the first couple of days with him and they then went to stay at grandmas for a week to give me time to sleep, establish breast feeding (I had a c-section so my milk came in later and I am unable to properly care for ds 2, the non-walking 20 month, by myself as I can't lift him)
-I have help starting next week. Our babysitter is helping out with ds 1 going to his mates to play, days out and taking ds 2 to nursery.
-I have said YES to any help offered and made sure there were jobs they could do. My best friend came yesterday and sorted out the boys rooms for when they get back, made their beds and tidied + did some washing. This helps so much.
-Try and identify what form your depression takes if any. Do you have triggers? Mine is anxiety based and I clean like mad when anxious and I get very stressed with untidiness. Dh knows this and we have devised systems to help us both, eg we leave a washing basket upstairs which I sort the laundry, he then takes it downstairs and puts it in the machine. I put the clean, dry washing in another basket downstairs, he takes it back upstairs.
-SLEEP -dh gets home at 7ish and I go to bed at 8 (we did this with ds 2 and are starting to do this again) he feeds ds 3 a bottle at 10pm (formula/ebm) and then I do the rest. This enables me to have 4 ish hrs sleep and then another 3 ish. Sleep is important for me as I literally can't mentally function without it.
-I also have a sling, a hugabub. I can't use it at the mo but after 2 weeks I can and will be all the time! Ds 2 loved his.
You will cope (I keep telling myself...) but ask for help when you need but keep talking to those who can support you about how you feel.
Good luck!