Needless to say, if a reason for love vanishes, then we fall out of love.
Love = Choice + Reason
The absence of one in that equation negates love.
And with this in mind, let me tell you the formula for a long relationship. Be consistent with who you are. Change confuses the reason. Always remember that moment when you fell in love with each other. I am not talking about the time when you felt the physical attraction, but the moment when you knew he or she is the one you want to live your life with for the rest of eternity.
That was you whom he or she fell in love with. That was the reason for love that he or she made the choice with.
Separations happen when one of you no longer see a reason to choose to love the other. It is when one of you has changed and the other doesn't see the perceived "real you" anymore. It makes letting go as easy as saying it.
So be the person that your partner fell in love with and stay that way. If it is not the "real you", but you want to stay in that relationship, then by George, train yourself to remain that way, and in time it will become the "new you"!