Y`know, Tectt adds another really great point to this topic! There`s the "let-down" affect, and maybe the best example connects to falling in love.
It`s well known that there`s an initial phase when we meet someone attractive, start to fall in love, and enter the so-called honeymoon period. There`s this giddy build-up of expectation, anticipation, mystery, and excitement. We begin dating and learning about each other.
But after a few months we experience a sort of slowing down. Depending on the amount of adrenaline and oxytocin involved, it feels like a let-down. Many cases of "love `em and leave `em" are associated with that let-down effect. The expectation of meeting The Perfect One can`t sustain the gradual coming in to balance, and it "feels as if" there`s just a big disappointment.
Think about it: Doesn`t this in some ways describe what you`re going through? Could it be possible, just as Techtt suggests, that you`re starting to feel the reality of your new relationship---falling in love with your own business and the future freedom and control over your own destiny?
Maybe now that you`ve "moved in together," metaphorically speaking, you`re just seeing all the little foibles and annoyances. If so, then it`s a matter of refocusing on the larger values. Interesting idea, though... :-)