I found that trying to be anything other than what I really am is a waste of time and just makes everything worse.
There's no other way than to be honest and be vulnerable and let her love you and help you through. In a polyamorous relationship you get to be loved as well you know, especially when you need it most. Does your partner understand that? Try and work out what is at the core of your upset, bring it up and work through it with your partner.