Brooke Chandler, a character in The Devil’s Light (a spy thriller with romantic undertones by Richard North Patterson) espouses the Chandler Theory of Romantic Symbiosis. The theory states, "that who you choose to be with helps define who you are and each couple created is a different entity.”
This theory stuck a cord within me, and I believe it can help you to adjust your perspective about your past relationship(s).
When you are in a long time relationship with a mate, you tend to emulate the characteristics of each other … moving towards the mean, if you will. Compromises help you to find new ways to approach situations, and sometimes this is a foreign method to you. (An added bonus is that you might even like the new method more!) You are also exposed to different hobbies, interests and various groups of people than you would have encountered if you had not met this exact mate. This, in turn, also shapes you and affects the timbre of the relationship.
In essence, your last partnership was unique – an entity onto itself which cannot be recreated, if one of the partners is missing. Consequently, if you try to duplicate this relationship, you will most probably be met with disappointment.
As a singleton, freed from the bounds of a past relationship (and with new-found knowledge and insight) you metamorphose into a different person. When you meet another mate, your new relationship becomes a different entity than your past relationship(s). This can allow you to let go of any guilt you may be feeling of leaving your former relationship in the past. That is the only place where it can reside, for it is virtually impossible to recreate it or hang onto it if the two unique members aren’t present to make it come alive.
In general, if human beings are to always be reaching higher – a work-in-progress evolving as insights are gleaned – then we as individuals may even defy the bounds of definition. This elusiveness, as we change by ourselves or when we merge with another soul, can allow us to remain in flux to maneuver the ups and downs of life and to have the appropriate tools on hand to successfully adapt to the myriad of circumstances with which we are presented.