"I need another option if he chooses to stay with her, not move on."
Is it possible he can choose to stay with her, *and*
choose to stay with you at the same time? In polyamory, we like to say that choosing one person doesn't have to mean "dumping the other." Doesn't mean literally being with both people all the time, just means that time is set aside for both people individually in turn.
"I've had some mental health issues as of late that I think he says what he is supposed to say to make me feel better. But I have read his emails to her, and they say a different story. He would rather be there. He doesn't say that outright, but deep down, I know it."
So you have concerns that he is perhaps not "playing straight" with you.
Let me ask you this: Why does he spend time with you (at all) if he'd rather be with her? What's holding your marriage together?
No offense (nor rhetorical questions) intended, just honestly seeking a gradually better understanding of the situation. Let me know as much as you can of what you believe is happening.