I need some advice on how to be happy in a relationship. (I'm probably not the only one)

I think it's a common story. I'm at that awkward mid-20s age when relationships are just starting to be judged as marriage-worthy or not. I'm with a lovely guy, but we see the relationship very differently. He's got that one-true-love outlook. I'm the first person he's ever really cared about deeply, and he thinks it's going to last forever. He's started telling me that he could spend the rest of his life with me.

I've been in love a couple times already, and while this relationship is great, it's definitely not what I want for the rest of my life. I just want someone to spend time with and care about, without the added baggage of "true love" and "forever" just yet.

My problem: once I fully accepted in my mind that I intend to end this relationship eventually, it's been a lot harder to get really caught up in the joy of being together. I feel distant.

Does anyone have advice or wisdom to share? How do you enjoy your time with someone, when you know you will be happier with someone else someday? Is there a good way to talk about this with a partner? I can't think of a polite way to say "I love you a lot, but not THAT much."