I see this situation a little differently based on my own experience (I have publications and have given numerous presentations). When writing publications, I WANT my mentor to shred it apart. It doesn't matter if I agree with it or not, it is necessary to teach me. I believe you are looking for a pat on the head when the point of her mentoring is to make your work better. This will prepare you far better than simply giving you positive reinforcement and saying that it's perfect. The reality is that drafts are almost never perfect and you will have to take a stand for what you believe it. On my last presentation, my mentor at the time gave me advice and he kept wanting to insert things or change things. I finally told him that his idea was very helpful but I respectfully disagreed.
Furthermore, she is preparing you for reviewers which can be ridiculously harsh on your papers. Understanding your needs versus what her role is as a mentor would be helpful for you to not internalize the criticsm. If you need positive reinforcement and encouragement, you should ask her for it. Ask her things like, "What are the stregnths of this paper/presentation?" Be upfront and tell her that understanding the stregnths as well as the weaknesses is valuable for you to improve and maintain morale. If your morale is dropping because of her harshness, use that as an opportunity to assert yourself and tell her. I've had to do this before. I worded it something like, "I appreciate all of your effort as it is helping me create a quality paper. However, some of the comments seem harsh or overly critical to me which is affecting my confidence. It would be helpful if you could strike a balance between the positives and negatives."
She's a new mentor so she is relatively inexperienced. She is probably completely unaware of how she is coming across. You could significantly help her and help yourself by providing her constructive criticsm.