Marriage.
She is my everything, and though as a GF she was that too, marriage is just deeper.
Many of my single/unmarried friends don't have the trust and support that I have. I try to tell them that 'pulling the trigger' is not with a gun aimed at your head, but more of the starter's pistol in a 3-legged-race; it's fun. Yes, we have issues, just like all humans. But as we have both grown and changed, holding each other up is something I can't believe I used to not have.
Many 'millenials' seem to view marriage as a capstone to their pyramid. Get 'a degree', get 'a career', find 'the one', fall 'in love', etc. That's wrong, marriage a cornerstone you build a life together on top of. And yes, I am lucky, I know it, and I love her even more as a result.
Maybe readers can extend this to any committed relationship, but I highly recommend taking the time to find that person and jumping in when you are only 70% certain of the commitment ( about 1 - 1/e, for the math folks). Don't wait, just love.