Perhaps it was "love at first sight", but then you get to know him/her more intimately and realize your goals and amitions couldn't be further apart. Not that he/she isn't a good person ... just not right for you. The physical attraction is real, but issues that are important to you pale into insignificance in his/her mind. After giving so much of yourself, he/she now holds a special place in your heart. You love them dearly, but they don't fulfil your deepest longings. You always feel there is something missing in the relationship.
What would be your choice? Would you stick around trying to change him/her and hope for the best? Would you be brave enough to forego the security of familiarity, to find true fulfillment in the unknown?