I was single when I had my youngest so I really do understand, but it is doable
Is your eldest eligible for nursery? My DD wasnt old enough for nursery until the baby was 5 months old. It was brutal on my own. By the time baby was a year old it was still very hard so I put the baby in nursery 1 day a week so I could have a break, mentally more than anything
Bath times on his own when baby Is asleep, colouring next to you or you reading to him if baby is asleep on you,
Simple things like when your popping to the shops, buy eldest DS a treat, tell baby he cant have any as hes not a big boy like DS yet,
Sleep overs in mummy's room.... DD loved this
My DD loves it when I sing silly songs about her, yank that oh Micky your so fine. I changed it to Oh DD your so kind,
Special handshakes ect, honestly any small thing that just you and him can do that baby cant, they love it and it makes them feel so big and special
You sound so defeated and worn down, do you think you could have a touch of PND? I did and didnt realise until I got better how bad it was. It was like being in a fog I didnt know I was in until the fog literally cleared one day,
Honestly, you HAVE to find the time, regardless of how impossible it feels. Small little gestures mean so much to them at this age