I like it when students tell me that they want to "do better." As a matter of fact, "do better" is one of two most often given responses (along with "get a higher grade"). I like "do better" because it shows a glimmer of growth mindset working in the student, and having a growth mindset is an advantage for those seeking greater satisfaction and success. However, "do better" is not specific enough for students to get the support, guidance, and help they may need to achieve "better."
Students need help clarifying "better."
One way to clarify "better" is to guide students through creating a vision of success. Ask them what success looks like if there were no barriers to achievement. See if they can describe a vivid picture of what they are doing when they are doing "better." Then, ask students to list all the barriers they see to getting "better." Finally, work with them on a realistic plan to overcome as many of those barriers as possible.
Also, it is helpful to set up specific points to use as benchmarks. While setting up fixed points to use as a measure of progress may sound like a fixed mindset, unless you are defining success by reaching those markers (instead of recognizing success through the effort needed to reach them) you are still focusing on growth mindsets. Establishing specific points will help define "better" in concrete terms and serve as a foundation for both students and those helping them. I tell students all the time that it is hard to hit a moving target. Define the benchmarks and stick with them. Then, everyone on their "success team" is able to draw conclusions and provide support towards a consistent point. If the "marker" moves, it is harder to support the student.
Again, setting up benchmarks is not the same as valuing a fixed mindset. Rather, having specific points along the way helps reinforce the effort needed to continue progress. This is especially true of new or difficult to reach benchmarks. Knowing where to adjust your efforts is an important part of realizing a vision of "better."