It's not crazy to feel that way. It's a mistake i think every Human is guilty of.
There's nothing i can say that doesn't make me sound like a boring adult, but it's the reality of things.
The odds of the two of you feeling that way forever are slim. And yes it's because you are both young. One of you will eventually reach a point where you feel you need to get more experience and not want to have been shackled to the same person their whole lives.
Thing is you are both still growing and maturing in the mind. We change a lot from that age, how we feel and think from our experiences.
When you're 25 like me you'll have felt like you've lived several personalities. 13 year old me is like an entirely different human being. Even my preference in girls has changed over those years.
I'm just saying you have a lot of growing to do still, and with that will come a lot of changes you have little power over, which shape your out look and expectations from life.
I guarantee they conflict with long term relationships, especially when you're younger.
You might not even love her. Sounds harsh. But when i was that age i used to think that. But when i compare those times to my adult relationships i realise i didn't know what love really was. I thought i loved these people, actually i was just highly fond of them.
Still, treat each other right and make it last as long as possible. Nothing wrong with that at all. I'm just warning you to not get so invested in the "spend the rest of our lives together" thinking. It's rare in even adulthood.