How to be more carefree?

me as a person I stress over everything including little things like “what backpack should I get for school” or “I accidentally bought 2 of the same item” whereas my brother who grew up with me somehow is so unfazed by everything. I’ve heard the saying “Unless your dead everything has a solution” but how do I stop always worrying and stressing myself out, I’ve gotten a bit better at it but I still tend to overthink, tend to think negative first/care abt what others think and overall stress myself out