Em, rather than being a failure, mothers like you are doing a better job than most mothers - because the task you have is harder.
And when you do make advances, they feel so much better because they have been well-earned! Some kids just take longer to work out the whole sleep caper, and some don't need as much sleep as others, or need a bit more help to get there.

For me, accepting that my DD was a minimalist sleeper made a big difference. I was able to stop struggling so much and work with the circumstances that i had.

Have you tried feeding her to sleep on your bed during the day? You might get a rest too, or if your little one sleeps you can sneak out and have a cuppa.

Nothing wrong with co-sleeeping if it gets you more sleep.