"We've agreed to make the most of what time we have and then move on. My friends have said this attitude is selfish and shallow of us, but I just don't feel that way. "
The first thing you should know is your friends are bullshitting you. Fuck them for saying you're being selfish and shallow. If you're agreed, you're agreed, why should they comment so negatively on a contract between adults in love?
That being said, I'll give you my opinion. Why think about four months from now? Why think about conclusions and endings? IF you think that way it will end and it will always end. It seems like your apt at living in love in the moment. Keep it that way then. Move from today to tomorrow to four months from now one day at a time. Listen to your feelings the entire way. They might say something new a week from now. But if you are still tuned in to this idea that your relationship will end, you won't be available to hear the new soundwaves that are telling you something radically different.
"Is the book any less meaningful when you know how the story ends?"
You don't know how the story ends. You're making it happen this way. Just as you can make it end any other way, like you could have made it end a week ago or a week from today on an airplane in a blue bonnet. As to the question of meaning, you won't be able to find an answer here. You've given us a fragment of your relationship. Even with more information and context meaning will always be yours and his. Every book is different, the end is always gotten to by a different channel of words, sentences, and chapters. Like any author, the meaning you get out of your own book will be different from the reader's.
"Most romances end."
Don't think about romantic demographics. Most romances don't have shit to do with you and your Kentucky beau. The future is an incalculable plane. Stop trying to see how it ends and start feeling how it's going.