What does "I can say it or I can eat it" mean?

I'm reading Year of Yes: How to Dance It Out, Stand In the Sun and Be Your Own Person by Shonda Rhimes (Simon & Schuster, 2015). She used to be unwilling to say no to anybody, but tries to change herself, and becomes stronger.

I laughed more. I was bolder. I was brazen. I spoke my mind and spoke it loud. And as busy as I was, I felt like I had more free time than ever; I realized I'd been wasting a huge amount of time and energy on complaining and feeling sorry for myself, being dark and twisty me. Now I wasn't interested in being that person. Not when it was so much easier to just open my mouth and talk.

yesyesyesyesyesyesyesyesyesyesyesyesyesyesyesyes

I've become kind of obsessed with difficult conversations. Mostly because of how calm life is when you are willing to have them. Also because of how much easier it is not to eat the cake when I'm not stressed out or holding a grudge or full of hurt feelings.

I stuck a Post-it on my bathroom mirror that says, "I can say it or I can eat it." As corny as it sounds ,it's true. I wish I'd learned to say yes twenty-five years ago. Between dieting and never saying what I thought, I wasted a lot of time.

I don't quite get what I can eat it means here.

I think by "eating it" she means to completely forget about what she might have said and let it go. The entire phrase "I can say it or I can eat it" means: either say what you mean, completely and unambiguously, or don't say anything at all.