I wouldn't say "ruined". You're too hard on yourself! Listen to your own description of yourself. Maybe you need to give everybody a break.
I recently found out that my 4 yr old got it into her head that I don't love her when she is in time out. I told her NOOO! I love her all the time, just don't always LIKE her behavior. Oy.
So....I'd sit down with your DD and apologize and thank her for being honest. Then I'd work with her on how to make homework time better. Is this something where you review it while she's not in the room? Where someone else handles homework? Where you sit down and work on being calm and make it your homework to work with HER as HER needs require?
My SD is good at math, but when she was in 3rd or 4th grade, you wouldn't know it from the battles at the kitchen table. The way my DH thinks is just not the way SD thinks and they would fight. I got involved and I was able to explain the math to SD in a way that didn't get anybody upset and in a way she understood. It was sooo much better for all if I did her math with her vs DH. So that's why I suggest if someone else might be better, to try it. And SD was stubborn. So if she really didn't want to correct it, then she owned that bad grade that she got later, and she knew the long term consequences of bringing home a C or worse for the marking period. Sometimes you just need to back off and let them deal with their own choices.
Please try to remember that everybody has faults and talents. I am no better than my sister who was never in GT. In fact, I think she's more talented than me in many ways.
Praise both kids for doing their personal best. If she gets As and Bs in her level of class, then that's terrific! And if he gets As and Bs in his, that's also terrific! It doesn't dilute her A if she's "only" at grade level. She's still working hard and doing well. Yay for DD!
Also look for other talents. My sister is very artistic and I'm about stick figure level. I commissioned a painting from her because I think it's just that cool. What else can you focus on with your DD instead of comparing her to her brother?
My SD has a high-achieving older brother. I actually had a teacher at the Open House her 8th grade year ignore me until wanted to ask how SS was doing. I was floored. I wasn't there for him! He graduated! I was mad on SD's behalf. SD ended up going to a different HS than he did because she wanted to be her own person and hated being compared to him. She's not going to an honors college. She didn't get a big academic scholarship. She didn't take as many AP classes. But she IS going to college, she IS getting a good education and she IS doing what she's good at. We're just pleased that she's doing what's right for HER.
So try to let DD be her own person. Tell her what you wrote about her art and how you love her and mean it. Do something together that has nothing to do with grades.
And remember, we are all just human. Dust off. Try again. NO parent is perfect. Not a one.