Melissa W. answered 06/04/19
Experienced Homeschool Parent with Bachelor's degree in English
While the usefulness of these suggestions would depend on the context, I hope that, if nothing else, these help get you closer to the perfect phrase:
"Have a little faith."
"I'm serious."
"I mean it."
"Seriously!"
"Honest!"
"Have I ever lead you astray?"
"Would I lie to you?"
"Trust me." (shortened version can seem a bit less drama-y, but still impactful)
I hope that these different ideas help. :)