I don't think making Mallory stronger is the issue, but more varied. This sounds like a one-note character to me. What happens after she always reaches a point of despair? Somebody rescues her, or the situation resolves itself (like the son returning of his own volition)?
That would not be a good protagonist, that is a punching bag. A good protagonist takes action, even in despair, because there is always one more thing they can give up: Risking their life, for example, to make the situation change. They cry, collapse in a heap, then get up and do the next thing.
The arc of most characters DOES reach a low point in each story, but they must pull themselves out of it; by perseverance, or by some earlier investment (seen by the reader) (a kindness to others, a message for help resulting in the cavalry coming, etc) coming to fruition, or a wise preparation or just-in-case plan (seen by the reader) coming in handy, or just plain innovation (McGyver or Indiana Jones style).
Reaching the low point is not the problem. It is very important how the main character gets out of it.
Your second problem is your publisher. Even if despair is a fine low point for the first book, your publisher may turn down the second book if it looks like the first book; most do not want you to write the same story over and over again, they want something new. I would be worried if the low point in every book is exactly the same, the path TO it is the same, the path OUT OF IT is the same, I would feel like I read this story with this character already. What's NEW?
In the first book it is okay, we are learning new things about Mallory all the time, and she is a well defined character by the end. In the second book, we already know Mallory so as readers we aren't learning much new, and if the plot is pretty much the same the book becomes predictable and boring. Publishers won't publish a boring book, it is not easy to publish a good sequel for exactly this reason: You have to take characters readers already know, so there is very little the author can write about readers "getting to know them" or the characters "getting to know each other" and a greater emphasis on them doing interesting things to carry the story along.