Hugs to you hun!! I know how exhausted you must be

but hang in there!!
At the beginning, shpat is very very difficult and tiring. You spend a lot of time leaning over the crib!! WHen I started shpat, it didn't work all of the times and it drove me crazy!! I understand what you mean about feeling like a failure, but believe me, you are not a failure, you are doing a great job!!
I would suggest that you get a 2 day vacation from sleep training. Do whatever it takes for sleeping your LO (rock her, feed her, whatever) and after these 2 days, come back to sleep training with a fresh mind. I did this when I was about to lose it, and it really helped me. I used to cry all day because I couldn't make my DS sleep, so I decided to take a break, and it made me feel better.
I started shpat when DS was 7 weeks old, then stopped it for about 2 weeks, and then came back to doing it. It was until my lo turned 3 months when I saw a big difference. I think it is something developmental. You help them, but they have to grow!! It is biological!! I guess that what I'm trying to say is that you are going through something very common and normal. Don't give up!! You'll get there, it is all a matter of time!!
One more thing...MY LO hated the patting on his back, so try experimenting with different things. My DS likes the patting on his bun, so try different things!! And don't stop shushing as you lay your lo down.
Let us know how things go
BIg big hugs

Alexa