*hugs* I agree that if you haven't been able to potty train her yet, she does need to go see someone. The worst thing that could happen is that they can't help. The best thing that could happen (and the most likely thing) is that they can get you both on the right track.
I agree with the other comments - definitely seek out some help. It's not you, and I can only imagine how insanely frustrating that must be! What does your pediatrician say? At our yearly well-child visits, our pediatrician always asks my kids if they have any trouble with peeing or pooping and if they have any accidents. I'd start there - tell the pediatrician and see what he/she suggests. I used to have a student who had a very mild case of spina bifida who pooped her pants and she was in kindergarten. It happens, for various reasons (obvs your girls don't have SB I'm just saying, there can be multiple causes.)
Your not a crappy mom by any means. We all have our breaking point. I have an 8 yo who still wets her pants. Nothing medically wrong with her.....she even does it at home. I think she just doesn't pay enough attention to her body. She doesn't wet at night...so the ped doesn't want to do anything about except just wait it out. I personally feel she was just never potty trained properly (she is adopted from a family member and I know how she was treated). I spend over a YEAR working with her being patient and setting alarms.....all to no avail. I am incredibly frustrated as my 5 yo has never had any issues with this. Although, my 5 yo was raised in a home with 2 parents and has only ever moved once....where our home for the adoption for my 8 yo was her 30th move since she was about 13 mos old. We are learning to deal with the issues, but they are no less stressful and frustrating.
My eldest is on the spectrum. He was able to potty train for pee fairly quickly...within a year or so, but we had issues with poop for several years. It does get better. Once he developed mentally a bit more it happened. I had to just back off on it. I had him teach his bear to use the potty, every single day. I had him go to the bathroom with me once per day (usually first thing in the morning). And I put kid potties in every room. It grossed me out since I'm borderline OCD but I tried to circumvent the dirty pants issue. It just took a loooooong time. HUGS,