This is a little hard to answer because it depends on what I am feeling not-assured about!
I feel reassured when a fellow Christian reminds me of biblical truths.
I feel reassured when someone tells me they love me.
I feel reassured when someone who is an expert tells me there’s nothing to worry about/something is fixed or handled.
(For instance, a doctor. Or a mechanic.)
I feel reassured when someone tells me I handled something the right way (I can tend to second-guess myself!)
I feel reassured when I tell someone about something that’s bothering me, and they say, “It’s going to be ok because _______.”
Sometimes, I can manage to reassure myself by looking at something objectively or by looking back at my life and thinking, “Ok, I made it through a situation like this before.”, or, “Ok, I handled this problem wisely in the past.”, or, “I thought problem X would go on forever, but it didn’t, so maybe this problem will also not go on forever.”