Ooh, that sounds familiar. My son was always perfectly able to do math, but always believed he was stupid at it, and balked at anything new. Then, of course, he would forget everything he'd learned before, claiming "see, I told you I was stupid!"
We tried various programs and approaches, including just STOPPING formal math altogether for a good long while... that was his grade 3 year. This was fascinating, though -- we were doing a CAT test at the end of the year, so I bought a little "homework helper" kind of book for grade 3 math, we worked quickly through it in about 2-3 weeks, he found it easy, and his achievement scores on that test showed MORE progress in math over the previous year than average... WAY more lol... That gave me a lot of confidence that taking time off formal math does no real harm!!!
Anyway, once we started again, we had the same "I'm stupid" problem come back. We were trying Saxon and that probably didn't help. I know it's perfect for some kids, but it wasn't for mine... He hated it, fought against it, and in the end didn't really learn from it. We quit about 1/3 way through, and took some more time off.
The first thing that really broke through his hatred/fear of math was Teaching Textbooks. We did their grade 6 book in his 'grade 5 year', based on their placement tests. He loved it... it's computer-based, he could work on his own, it had friendly cartoon helpers and clear explanations, and a 'second chance' if you made a mistake.
He finished the year with a 91% average, newly found confidence, and an eagerness to do more math! Unfortunately, he also had very low retention (the format made it easy to do well without truly absorbing the information), so when we tried going on to pre-algebra it quickly fizzled... he obviously was NOT ready.
Our next experiment was RightStart. He tested into level E, so we went into that after doing the Transitions lessons. This finally got some real comprehension and retention! He loved the games, the abacus is great, the method is fantastic. He just finished that about a month ago, and we've started their Middle School program, A Geometric Approach. He loves this best of all!! He's even done some lessons on his own, JUST FOR FUN.
The final experiment with great results was Life of Fred. We don't use this as a stand-alone program, but as a supplement. He LOVES Fred's stories, so it keeps him engaged. It's great extra reinforcement.
We have used Math Mammoth a couple times when he needed targeted extra practice on a specific area. Very convenient for that! Spend $3 for a workbook just on improper fractions or whatever... lol... I PERSONALLY wouldn't suggest it as your main standalone program (though again, many people do love it for that), I found it a little dry and uninteresting -- and it's all just off the page, there's no hands-on exploring -- compared to RightStart.
So, long story short -- I'd look into Teaching Textbooks and RightStart (given what you said about what she can do already, I'd suspect she'd be level E), and Life of Fred.