Peggoty Just to add my support/sympathy. it IS a really tough time.
Something that turned a corner for ds and me when dd was born was me making time for him on his own. I would leave dd with dh (or PIL/mum/anyone who'd have him and go to the park with him for 30mins. Got us both some fresh air/something different to do, reminded me why I liked (as well as loved) him.
Like marmaduke I had a 'treats' box for ds which I could dip into whenever I needed - I briefed family (who, like yours don't live locally) to look for small gifts, wrap them up & send them to me. Then when dd wanted a feed and ds didn't want attention, I present would be produced. By the time he'd opened it & figured out what it did, dd would be sorted
Take any help that's offered. If its not offered ask - eg if famiily come to visit, tell them to bring lunch, ask them if they'd do some hoovering/ironing. They will say yes. They will not be offended. Bet you
And sleep, sleep, sleep, Put any household tasks on a 'low priority list. If you are all dressed & have eaten something, you have had a successful day
And, as other people have said, find creative ways to involve her. I know, creative ain't where you're at right now, but look at what she is interested in with your baby. For eg ds ADORED the fact that dd did yellow poo "look mummy, poo like the sun!" SO, whenever we changed nappies we would discuss excitedly how yellow the poo might be. Someone wise said to me that toddlers/preschoolers don't find babies loveable, but they do find them interesting - we found we could really use that....
Good luck - it WILL get better