'What it's like to have a baby' is a bit like 'what it's like to have a mother/boyfriend/husband/sister' - highly individual, changeable over time, a nightmare for some, a dream for others!
Yes, it's hard work and incessant. I forgot what having a baby was like in between DC1 and DC1 and they're only 2.5 years apart! It was only when DC2 was born that I realised how utterly individual babies are - they were very different experiences. DC1 was colicky and had trouble feeding, always quite demanding - DC2 just wanted to feed and smile all day.
In your shoes, I'd talk to DH and get him to give you a break more - start with him having DC while you have a bath/go for a coffee/go for a walk and work up.
Family and friends with older kids - they're not mind readers. If you're waiting for offers of support, try saying 'I'm finding it hard, would you ever be able to watch the baby for a few hours while I get my hair cut etc?' or 'if you ever want to babysit and have some cuddles, just let me know!' Sometimes people would be happy to look after a baby for a bit but think you'll imagine they're implying you can't cope if they offer. On the other hand - did you offer to help when their older kids were small? And older kids can take more time than you imagine with ferrying them about etc.
It's true though, people can't understand what it's like to have kids before they have them, any more than telling someone what it's like in a coal mine prepares them for being a coal miner. Because it's your specific emotional response to lack of sleep and lack of freedom to do what you want, when you want that matters, and it bothers some people more than others/is harder with some babies.