I have a 7 year old son who has the same issues. He's actually convinced that he stinks at math because he continues to have to practice his "math facts" (addition, subtraction, multiplication), yet he understands long division, geometry, fractions, etc. So, he gets the big concepts, but he can't stand the repetitive practice that memorization requires. Same thing with spelling. He thinks he's terrible at spelling, yet you can ask him about any phonetic spelling rule, and he knows it. He just can't seem to memorize the words that "don't follow the rules".
He gets so frustrated and has major meltdowns when he can't do something right away. He's gotten better with it, but that's not saying much. Where he used to refuse to try something more than once, he now will try a few times before losing it.
I wish I had advice beyond what you've already been given, but honestly we've found nothing that works really well, at least not right away. I do agree that modeling good responses to frustration and practicing (together) activities that require repetition, practice, and multiple attempts make a difference. We try to manufacture scenarios where we fail so that we can model the right way to handle it. We ALWAYS point out our mistakes and problem solve aloud. We try to limit the amount of time doing an activity that we know is going to trigger frustration. For example, ds often melts down when playing a video game on the DSi or iPhone. So, we give 15 minute blocks of time to play. We still get a small meltdown when we have to take it away, but it's nothing compared to the emotional response he'll display if he plays the game too long without enough success.
I wish I had some great answer for you. If you find one please shout it from the roof tops, because it would change our world! This issue is the root of most of ds's issues and is affecting him socially and psychologically (becoming depressed). It's amazing how it affects his self esteem when I know he performs so far ahead of many of his same aged peers. He tends to focus on his difficulties rather than his strengths.
I did get a book called, Don't Pop Your Cork on Mondays and another called, The Gifted Kids Survival Guide. They have both been helpful for our son. I find him in his room reading these books to himself. Also, he recently began seeing a counselor for the frustration/meltdown issue (doctor thinks that they are somewhat like panic attacks because of the physical response-rapid heart rate, throat closing, difficulty catching breath, muscle tension), and ds reiterated ALL of the methods he learned in the Pop Your Cork book to the therapist. I was proud of him.
That said, the therapist and I asked him if he thought he could try using these when he has a meltdown, and he said, very matter-of-factly, "no, once I'm having a meltdown, I can't stop it. It's too late."
My hope is that, although it will take time, these efforts will make a difference in the long run. It's a tough one, and I have to say, I still have trouble with being hard on myself for making mistakes. I have ADHD just like my son, so I make lots of them! ; ) I have to be very careful of my responses, because I'm famous for saying, "I'm such a ding-dong sometimes! Why do I do things like this?!" Those are the responses that I MAKE myself avoid in ds's presence.
He's also 7. What's worse, he'd be so rude when he asked for help. Took me a while to figure out that I should disappear when he's doing his work, so when he gets stuck and really needs help, he'd have to ask nicely. 