Hi OP, yours is very much a taboo subject. There are a lot of mothers who feel the way you do..just its not a 'done thing' to admit it. Men who realise they have made a monumentally bad life decision and walk away aren't vilified in the same way as women.
I haven't read all the posters comments so I'm sure there is some good advice.
I don't know what your child is like obviously but I do know from bitter experience that's not always something they outgrow...I spent days hours planning how to escape from it..
DS1 was like it, also slept 2 hours in every 24 cat napped during other times, long enough to recharge but not enough for me to recharge, I was told he will get easier at -insert point- weaning, crawling, start walking, playgroup, school....he did improve at 8 until 14 boarding school was the saviour there (scholarship)...structure, high academic expectations, 2 hours sport a day...fast forward to 14 it all started again..11 years of hell, major fails at life decisions including fathering DGC with an equally dysfunctional woman ...then diagnosis of ASD/MH issues through court reports...
I could have predicted all of this at 4 but was told 'he is only a little boy, he is extremely bright, he isn't that bright (he was extremely bright actually, genius level IQ, ) you have PND, it will get better'
Good luck OP....I hope you find a way through...FWIW my son and I have a very good relationship even though he has put us all through shit...and weirdly I do 'love' him even though I don't like what he does although if there had been a bigger gap between him and his siblings they would never have been born....