I would focus your work on her in sight stays. You want to be able to be about 30 - 40 feet away for about 5 minutes or more without having to go in and treat. The more confident she is on the in sight stays the easier it will be for her to transition to the out of sight stays. You want her to be more relaxed on the stays so make sure you are treating frequently and going very slow. Up your time and distance slowly, and not at the same time. Also make sure to very gradually reduce the treating. But for now I would be treating fairly often.
If she should make a mistake, just go back in with a smile and in a happy voice, say "ooops!" (if she seems upset by this don't use a verbal at all) and encourage her to get up and move. Once she has gotten up, set her back up for the sit stay and try again. Don't use the "aaahhh" at all, since this escalates her stress levels.
You're doing great by helping her be more successful after a mistake! Be sure to continue this. Stays are slow going even for very confident dogs, be patient and keep up the good work.
Dawn
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