I never understood the sentence "I have my heart in my mouth." Not until I tasted it, not until I spit it, not until the words got stuck in my throat because I felt a weight on my mouth that didn’t let me breathe.
I didn’t understand the sentence until I felt my chest empty and its beating on my neck. Until I cried because I couldn’t even talk.
I didn't understand what "Having your heart in your mouth" meant until I found it there and I had no one to turn to.
Hopefully 'I have my heart in my mouth' is an expression that IS actually used in english, because the original poem was about a spanish idiom ('tengo el corazón en la garganta') that IS quite common.