How long can you suffer from ppd

So i have a 5 year old and a 6 month old baby . First pregnancy was fine wasn’t depressed was happy nothing extreme,

This time with my second? Forget it. I had major anxiety at first and now I’m so depressed at times. Between juggling two kids and feeling like I’m failing them like i literally laid my 6 month old in crib with a toy so i could do dishes fast he ended up screaming, found him with vomit, i don’t think he choked on toy it’s not small at all !!! he threw up last night too, thinking it could be a stomach bug but i feel so damn guilty about it and I’m so upset. I’m tired of feeling like i suck as a mom or that i don’t deserve my kids or maybe if i was died they would be better off without me. I’m just a huge mess between juggling them, testing possible for thyroid cancer and then hating my job doesn’t help either.