In the space of five years, you've had a lifetime of experience with this person,...as far as excitement, they're a hard act to follow (and you've become accustomed to all this excitement as a "ho-hum" everyday experience, well, I can assure you it's not). After getting used to all of this, I could see how slowing down once in a while and taking a good look at reality would be disappointing (the result,...sadness, loneliness). He has raised your "threshhold of happiness/excitement/ satisfaction/pleasure".
But your "sadness" right now is not simply that you are noticing the contrast of what he has shown you for the past five years, it's that he has enabled you to look inside yourself and notice the dream of YOUR "heart".
I never really thought that the term "SOULmate" could adequately express a love relationship for the simple reason that love comes from the proverbial HEART, not soul, I think you sense that too.
I think that calling him a SOULmate is what you really find uncomfortable because, in order to call (and think of) him as a soulmate, you have to deny your heart