How have your long-term relationships changed over the years?

I don’t have good role models for what good long-term relationships are supposed to look like, so I spend a lot of time wondering what’s “normal.”

Can any of you older and wiser folks share some of your experience? I am especially curious about how relationships change—the kind of intensity and and passion you feel at the beginning of a relationship inevitably fizzles out, so how do you manage that transition to… whatever comes next? What does come next, anyway?

Beyond that, how did you sustain a long-term relationship? In your experience, when these relationships failed, how did you know they were failing? How do you distinguish between a rough patch and a red flag?

If you could go back and give yourself at 25 some advice about long-term relationships, what would you say?