- - Improve practice quality
- - Reduce frustration
- - Increase satisfaction
Take the piece or passage you are "failing" at and ask yourself why you're getting frustrated.
Is the answer something specific (like: "this section is out of tune", "My sound quality is poor"), or general (like: "This doesn't sound good", "There are too many mistakes")?
If the answer is general, pick a specific problem you can work on, then redefine what success means for that passage.
So instead of thinking: "Success = playing this page perfectly", reduce the difficulty to something manageable and change your definition to something achievable and quantifiable like: "Success = playing these 3 bars with a good sound".
If this is still too difficult, redefine even further to something like: "Success = playing these 2 bars slowly with a good sound".
Repeat this process until you can consistently achieve your new practise goal.
Once you've reached a point where you can consistently "succeed" the majority of the time, you can "redefine" in the other direction by increasing the difficulty slightly (e.g. add back vibrato, or the other note of the double stop).