Hi OP! I had severe PND with my first pregnancy and was very, very worried that it would happen again with my second but it didn't and I had the most wonderful maternity leave with her. It was night and day in comparison to when my first was born and I was astonished as it really brought home just how ill I'd been.
My son was three when his sister was born and the age gap is just perfect. He could do plenty for himself so I could focus on the baby without him needing me to feed him or take him to the toilet etc. He LOVES playing with her and making her laugh so having a permanent source of entertainment for her is a great help. He also helps bring me nappies and other things that the baby needs if I ask him.
I vastly prefer having two children to just one. Everything has felt so easy this time around as I already know how to change a nappy and make a bottle etc. I knew what to expect re night wakings and how best to handle the tiredness. Seeing my children together and interacting is just the best feeling.
My husband and I have just finished our parental leave now so we're in a new routine of getting the baby off to childcare, our son off to pre-school and ourselves to work. I was initially worried about how it was all going to fit in but we manage it between the two of us and it all just seems to work out somehow.
What time of year was your first born? I ask because my first was a late autumn baby and everything was dark, the weather was miserable, we moved house and it was also the third Covid lockdown so I couldn't go out or see anyone or go to baby groups. That, plus being badly injured, is likely what led to my PND being so awful. This time around it was late spring, everything was warm and sunny, we had plenty of help and I joined a baby group and made some friends.
If your baby is due in the summer even just a small thing as the weather being nicer and the sun being out could make a huge difference (as silly as it sounds).
Definitely talk to your midwife about all of this. I was referred to the perinatal mental health team and had plenty of appointments before my daughter was born. They were ready to help in case I had PND again.
Another thing I'd suggest is having your mum or MIL stay with you for a couple of weeks if possible to help with the three year old whilst you settle into life with the baby. This was incredibly helpful for us.