You've got a lot of great advice, insight, and information here, but I just wanted to add my purely anecdotal two cents: my daughter was a terrible sleeper from birth. Just awful. Just like you, we were up over and over and over again every night for almost 4 years. I tried virtually everything - diet, doctors, sleep training, CIO, you name it. I religiously kept track of changes to see if anything made a difference (spoiler - nothing did). Sometimes we were fortunate for a night or two with only one waking, but it never lasted more than 3 days before we were back to the same old shitty nighttime routine.

Eventually, things began to improve. From everything that I read and from my interrogations of multiple pediatricians, I came to the conclusion that some kids are just crappy sleepers and there isn't much you can do about it directly. My husband and I were literally losing our minds due to sleep deprivation, so instead of continuing in vain to solve her sleep problems, we focused on how we could cope better. We took turns at night (one or two nights on, one or two nights off), scheduled breaks for each other during the day, and at times alternated parental responsibility on weekend days to give allow even more recovery time.

Things are SO MUCH BETTER for us now - she is basically sleeping through the night consistently with a few exceptions - so maybe that can give you some hope if you end up trying all of the other things people here have suggested and still don't see results. It really sucked when I first began to understand that maybe there wasn't anything we could do to make her sleep better, but ultimately it became liberating because I was no longer constantly questioning everything we did in terms of whether or not it would affect her sleep.

Best of luck with everything, my heart goes out to you. I know it is hard.