We had my oldest daughter repeat 4th grade... she was very intelligent, but got distracted easily. However, we did it when we moved from one state to another, so none of the classmates in her new school would have to know. (She is an August birthday, and was always petite.)
For her, that was the best thing we could do. It gave her a chance to catch up, both socially and academically, without anyone knowing that she had been retained. She is now working on her doctorate in Microbiology at Johns Hopkins.
With daughter #3, we delayed her kindergarten entry 1 year... that also helped, but she still struggled socially. After some difficulties (mostly social) in high school, she is now working toward going into nursing.
I really see ADHD symptoms with #3, and possibly some with #1, but not the same way. They were never diagnosed officially. (I have some of the tendancies, also.)
But..... you already see meltdowns and her feeling that it is all too hard, and constantly failing. At some point, she will just give up. She needs to start having some success in school. When she works with you, do you see that she does know the material, or is she having a hard time comprehending what she is learning?
I really think you need to look at learning disabilities or something like that. As someone else posted, talk to the teacher and/or counselor.