Why???? For the love of God why?

I just don't understand how my son was able to sleep when tired as a newborn but for the past month I have to do everything imaginable to put him to sleep when he's tired. He's actually not bad when I put him down for bed but all during the day I feel like it's a fight. Then after his first long stretch at night it's a fight to get him to stay asleep but not to go to sleep if you get what I mean. It's just so hard and I'm responsible for his care no less than 80% of the time. I'm at my wits end.