The "breaking point" often comes from regularity of criticism, even if it is valid and non-personal
I don't think the issue here is a matter of criticism being "too hard" or "too soft". The line determining when criticism "becomes personal" is about the target of the criticism, not the intensity of the criticism on that target. If the criticism is focused on the content of your work, or even if it makes a comparison of the quality of your work relative to the expectation at your year level/cohort, then that is a valid pedagogical observation. Such criticism aims to give critical feedback on the content of student work and an assessment of where its quality stands relative to cohort expectations. On the other hand, if the criticism strays into territory where it is directed at the inherent qualities of the person presenting (e.g., their presumed intelligence, etc.) then that is when it becomes personal. This is true even if the latter kind of criticism is less harsh in intensity than the former kind of criticism.
One of the things about being a student, and being instructed by experts in the field, is that your entire education program consists of being given correction and critical feedback to gradually improve your work (and this is as true for PhD students as for early undergraduates). By necessity, much of that feedback focuses on the areas where improvement is required. The regularity of this criticism, and the fact that it lasts for years, can lead some students to feel beaten down even if the criticism itself is perfectly valid and non-personal from a pedagogical standpoint. In such cases, students need to balance out the regular criticism of their work by taking breaks and reminding themselves of how much they have improved in the years of their education. Ideally, supervisors and teachers will recognise this dymanic and also try to temper their regular criticisms of work with periods where they focus on positives, take stock of progress, or just offer students breaks in work where they needn't worry about improvement.
One of the great things that the Stoic philosophers teach us is that we need to stop always looking ahead to what we haven't got yet, and take the time to enjoy the things we have already achieved (I highly recommend reading Epictetus on this point; he will make your life better). For students (particularly those in the late stages of their tertiary education), this means that you should take time to remind yourself how much more you know that when you started your education, how much more profient you are, etc. It is normal to feel deficient when you compare yourself to professional scholars who are experts in the field, and to feel beaten down by the drum-beat of constant critique of your work (particularly if you stay in tertiary education for many years). Use this criticism to push yourself forward, but make sure you take the time to remind yourself how far you have already come. In my own supervision of students (whether it is undergraduates in courses or PhD students doing research) it is common that most of the feedback I give them is about deficiencies in their work that they need to improve. However, I also try to improve student morale by reminding them of how far they have come since they started their education. You need to balance those two things to make sure you don't reach your "breaking point".