The easiest way to tell her is to just tell her. But consider what she is trying to express to you rather than focusing on the specific wording she used to express it.
Your mentor wasn't really commenting on your home practice of meditation, at least not directly (even if she expressed this poorly). It's clear that she expects specific results in terms of improving your attention span and mindfulness, and she believes that you have not achieved those results. She apparently feels that the explanation for that outcome is that you haven't been practicing (or have not been practicing very well), but this is not the thing you should respond to.
The on-topic, direct response to your mentor is to explain why she's mistaken about the results you've achieved, or offer an alternative explanation for why you've not reached the level she expects. The response you did offer, as described in the question, contradicts everything about what your mentor expressed: you claim that you are practicing (in the way she expects), but are achieving inferior results (compared with what she expects), and have offered no response explaining why her expectations are wrong or your performance better than she thinks. Assuming that she is a nominally rational person (she won't just make things up for no reason), the possibilities for explaining that can be grouped broadly into:
You are not practicing, and therefore are not achieving the results she expects
You are practicing, but incorrectly or poorly, and therefore achieving some results but still falling short of what she expects
There is something unusual about you, which she cannot perceive, which causes you to progress more slowly than a "typical" person would (that is, her expectations are wrong in your specific case)
She is not knowledgeable about meditation, and therefore is not equipped to teach you about it or assess your level of ability (that is, there is no reason to think that her expectations are realistic or valuable)
Given that she is, presumably, more of an expert on the topic than you (that's why she's your mentor, rather than the other way around), it's more sensible to accept her opinions than yours when they conflict. So (4) seems like a poor candidate to describe the situation. (3) is basically invisible, and if she's taught or mentored many people in meditation it becomes less and less attractive as a possibility (though it remains possible). (1) or (2) are attractive as potential explanations, if only because meditation is difficult.
If you are absolutely certain that (1) and (2) cannot be the case, then it's on you to express why you feel that your progress would be inherently slower than for others, or why her judgement is off. You'll have to explain why your practice has not produced the results she expects. Otherwise you're just meeting her (expert) opinion with your own (non-expert) opinion and expecting her to believe you instead of herself. That's a big ask, and an odd thing to demand in this scenario.
My own experience:
I've tutored many people in various subjects over the years, and I've almost never assigned an amount of time to spend studying. The criterion I judge progress on is whether or not specific learning goals are met, and if they are not, then the student has not studied enough.
Several years ago I was tutoring someone in Biology and instructed her to memorize the definitions of a set of vocabulary words. Those words were necessary to discuss the content she needed to learn, which was the real focus of the tutoring. When we next met and she had memorized between 5 and 10 of the 30 (or so, it was a while ago) words, there was no way to pretend that she was prepared for our session. I helped her review them, but that was an inefficient use of the time: she could have looked at flash cards just as well on her own, while she would have no chance to discuss and ask questions alone.
It would not have been appropriate for me to say that she hadn't studied at all, but clearly she had not studied enough (or efficiently enough) to stay on track with the schedule I'd planned. She'd made progress, but not as much progress as she wanted me to help her make. Had I said
You need to study more
and she responded
No, I study enough
she would have been obviously wrong. The standard was the outcome, not her effort, and I'd laid out that standard in advance. If the outcomes were falling short, more effort would be required.
Had she said
But I studied for five hours every day!
I would be extremely skeptical, not because that couldn't be true but because I would absolutely expect 35 hours of study to be enough to memorize those definitions. She could resolve that seeming disconnect by saying something like (as hypothetical examples) "I'm dyslexic" or "I have ADHD", something which would suggest that she might get less out of an hour of study than someone else. But absent a way to resolve my expectation with her observed performance, I would not believe that she had studied for 35 hours.