HaRD enough

During practice, there is too easy, too hard and just hard enough to be really making progress. Staying in that zone is tricky, but one sign post to look out for is the phase I call HRD. It stands for a "Halting Run-through with Do-overs" (thanks to James Stern for the term). It should sound familiar to anyone who has ever tried to learn some music. It means that you can get through the passage or section (or whole thing) but you have to stop and start and sometimes go back to correct mistakes.
Some might suggest that doing this kind of practice is not best, and that one should always be isolating problems and solving them, but I think it is a legitimate strategy, and not one that indicates wasting time. I tell my students that it is a sign that they can do it, but it is HaRD.

There is no reason to think that there is some way to practice slowly or carefully enough to completely avoid ever making mistakes. Mistakes are your opportunities to improve, and while they should never be ignored, neither are they an indication that you aren't practicing correctly.  As I work with students, I often allow them to work through a phrase doing HRD's. In fact, I will suggest doing one set of beads (five repetitions) in order to see if it is getting easier. They have to get all the way through the selected section, correcting any mistake, but then we move a bead. By the end of those repetitions, we are often already making it through without needing corrections. Often it is still halting, but perhaps the last repetition or two were without actual wrong notes or bowings. This is the next sign post I look out for, a HR without the D's.

Perhaps next we do another set, only moving a bead for HR's, not if there are any actual wrong notes played. By the end of these repetitions we are often able to do some without the stops and starts. Of course this is the real goal: playing through with no mistakes and in a consistent tempo, but it is not necessarily the first goal. So of course the final set of repetitions will only move a bead if it is of this type. Stern calls them an "old-fashioned recording session" or "record with no edits."
After this, of course, it is time to move on in practice. As you tackle new music, learn to recognize the signs of progress.