Why are you worrying about your underwear? Modern underwear, even the most inexpensive stuff, is not likely to just give up and leave you, unless you have repeatedly boiled it and dried it by firing at it with rock salt.
Perhaps this fear is one of those worries that masquerades as something else, otherwise it makes no sense. Assuming runaway underwear isn’t a real worry on its own, maybe what you are really worried about is something else, like discovering that you went out with your shirt on backwards. Or possibly some other potential faux pas is troubling you, like sending a deranged sexual text to your traffic judge. I get you. It’s hard to get everything right.
Everyone worries about looking foolish and some folks are completely paralyzed by this kind of fear. What if I stress-poop in the supermarket? How can I live with walking through the aisles while each person in turn groans and looks at the nearest baby? What if I burst into tears at the car wash, because I finally have some quiet time watching the soap bubbles desert me like everyone I have ever loved?
These are totally normal concerns. Don’t let the concept that you will look foolish torment you. Embrace it. You will shit your pants, you will cry when it’s inappropriate, and you will laugh at the very worst moment. If you don’t look foolish regularly, you really aren’t living your life.
Get out there and be stupid, but be kind to yourself and everyone else. Tell your crush how you feel. Buy the doughnut and then try to give half of it away. Tell a stranger they have wonderful posture. Cuss when you stub your toe and don’t stop cussing until it feels better. Create crappy art as long as it takes to make it a little less crappy. Show your crappy art to other people who will hate it and are certain to tease you about it. Fail like it’s your job to keep failing at things.
Sure, you’ll feel embarrassed at times. Being a person is embarrassing and only monsters don’t have the sensation. Find out if you are a monster! I bet you’re not!
When the embarrassment hits and your underwear falls off, don’t stop walking and don’t reveal any shame.
Remember: your ass has every right to be out in the world.