Things I believed as a teenager that I don't believe now:
* My friendships will last forever.
I always believed friendships don't end "without a good reason." But over the years, I've had many friendships end by just fading away. People move away and if the person doesn't enjoy writing or talking on the phone, it's almost impossible to maintain the friendship. I've read that most friendships last about 7 years. I'm still sad when a friendship ends but now I don't feel like something is wrong with me when it happens.
* Most adults are governed by logic, not emotion.
When I was a teenager, I thought if I just continued to improve my logic and reasoning abilities, I'd be able to have logical arguments with other adults when I became an adult. Nah. In my experience, most of the time, people don't care about logic or truth, they just want to win. And many people will lie, cheat, shout, yell, and cry to win.
* Health problems are the exception, not the rule.
I used to think that "most people don't have health problems" so any kind of health problem seemed unusual to me. Nowadays, I think most people have had at least one "health scare," and many have ongoing health problems. My friend's husband has multiple sclerosis (MS). He's a little clumsy when he walks but otherwise he looks healthy. Nowadays I think health problems are the rule (the majority of cases) and someone who reaches old age without a major "health scare" is the exception!