can love hit a permanent plateau?

[Moxie]

I've got a relationship question for all the love doctors out there.

My boyfriend and I have been together for more than 13 wonderful months. Even from the beginning, we both have attempted to have a "realistic" relationship. By that I mean we don't pretend that we are going to be together forever. While that would be lovely, we both know that it is an unrealistic ideal, and that by removing that ideal from our relationship, it will be easier for both of us in the end. We're both only 19, so obviously believing that we will be together forever is bordering on silly. I know that some people marry their high school sweethearts, but I don't know how common it is. I know that I've gotten a lot of slack from friends for talking openly about the possibilities of what will happen when we break up, etc. They are always like, "Do you want to break up? Then don't plan for it!"
It's never been that big of a deal until now, when I realize that our relationship no longer has any room to grow. I feel that because of these "realistic limitations" we have set, my love for him must plateau. I've been frustrated by this recently, because I think the would-be-growing-love has no place to go but down, so it is smushed into corners and sideways air pockets - making me SO frustrated! I don't know if this makes sense to any of you, but I tried to explain it to my boyfriend tonight when he asked me why I've seemed upset the past few days. He acted like he didn't really understand, told me that he loved me, and said that whatever plateau we had reached was a good one. A GOOD PLATEAU?! Blah on him. I think I kind of wanted him to tell me that I didn't have to limit my love. I'm just feeling that if our relationship is going nowhere, what's the point? He says that's not the case. But even though I love him and he loves me, I can't help but feel that we're headed into a big mess. It's making me anxious and I can't sleep, so I thought I'd put this out here to get it off my chest.

Console me, advise me, love me

(edited to add our ages...since i guess it's important?)