Well it's great you have a Tutor for her.
At 12, this is a hard age... they are changing so much, and yes, hormones too, for a pre-teen.
Is there anything that may be contributing to her lack of "ability?" Do you think it is TRULY her lack of ability or learning....or a lack of motivation? Is she generally a "happy" child and has normal friends/activities/social dynamics/etc.???
Your daughter, per your request, says that she just doesn't "get it...." and goes "blank." Perhaps... is there maybe something that causes her to not be able to concentrate? Math a lot of times is memorization of things and patterns... maybe, help her learn methods of remembering...or mnemonic memorization methods... etc. Some kids just learn differently... ie: some are visual, some are auditory, some of kinesthetic... etc.
You said she does not do her homework....is it just because of her lack of "ability?" If she is having trouble... I would speak to her Teacher(s), and seek advice from them.... or ask them how her behavior is IN school... academically AND socially...
The thing is, there is usually a way to help a child....if that solution can be found or the "reason" why can be found for why the child is having trouble...
Perhaps she just needs "help" and company while she studies... and with you? My daughter is like that, but she is much younger.
For me, I was never real "talented" when it came to math too... it took me awhile to catch on. No I wasn't "dumb" ...but my Mom would get frustrated with me too, because she tutored me (and she was a brainy Math major in college & a Valedictorian), and she never understood WHY I was not so "fast" with Math. Oh well. But, I excelled in many other academic areas. But, well, so I never was an "A" student in Math... but it was grueling. But I did good in Science. Just Math was my weakness. But I survived.
So maybe, just her "lack" of understanding Math, is now giving her a hang-up about other things... and making her self-confidence shaky in other areas. Maybe this is just making her not want to try anymore.
Try and see what her Teacher would suggest.... start there. Even at this age they are not "grown-up" enough to do everything by themselves. And just keep supporting your girl... at this age, they are also quite sensitive and really get affected by what others think of them... keep her confidence alive.
All the best,
Susan