4 month old won’t sleep during the day

My son is 4 months old and he will only ever sleep for 40 minutes at a time during the day.

At night time, he will sleep alone for hours and hours (only feeds once overnight) but during the day he sleeps for 40 minutes alone and will cry unless I rock him back to sleep and continue holding Him. I’ve tried everything to get him to sleep longer and nothing is helping. (Things I’ve tried: Keeping up longer/shorter amounts of time, baby cereal, formula, different swaddles, a safe comfort teddy, rocking him to sleep, letting him self settle, making him lots of activities so he’s absolutely worn out).

I run my own business and have a toddler, I’ve also got anxiety and my partner is concerned I have post natal and I’m just so over this. Its so hard to be positive when all day I’m fighting to get him to stay asleep. But the end of the day I just want to cry and for him to leave me alone.

It’s really getting me down because I never get to spend time with my toddler as the 40 minutes baby is asleep I have to do household chores. I’m sick of it. Help.