There's this concept in Merleau-Ponty's phenomenology of perception called "optimality." It argues that there is a particular concordance of sense perception, of bodily disposition, that works best in particular situation. An example would be of glasses: to a person whose vision is impaired, the impairment is the norm of their visual perception. It is not optimal for operation within the world because they do not have an accurate concordance of sense. However, it is normal for the individual in so far as the individual does not know a concordance of senses that is better for movement through the world.
When the visually impaired individual is given glasses, a new norm is put in place. We might call this super-normal. This new norm is optimal for moving about in the world in so far as the situation of moving through the world requires a particular concordance of senses, a particular disposition of the body. If the glasses are taken away, vision reverts to the previous norm and is thus less than optimal. This sub-optimality is perceived by the organism as a disruption in the sense experience, a disjunction in the world, and so the body continually strives to attain the optimum that it has lost.
Our affective sensibilities are like this. We can take the example of love. Once one has experience romantic love, a deep connection with another lived body, a new norm is put in place of the old norm. Depending on the depth of the affective relation, the love, the individual will look back on their experience prior to the relationship and wonder how they managed to exist in the absence of this love, this new norm. This new norm becomes the optimal mode of being in the world for the person in question, at least until the relationship ends.
In other posts on the subject, I have indicated how the severing of a relationship introduces a disorientation where a subject keeps reaching out into the world for something to arrest the affective freefall. Further, I've written on how the personal time of the individual halts because they are literally attempting to posit a future that simply cannot be. Here, I am prepared to argue that both of these situations are the result of being forced into a sub-optimal being in the world.
That is, in the termination of the relationship, the individual loses the optimality of the intersubjective relation with another person that enhanced and enriched their life. Their life was made super-normal in the presence of another, and when that presence is gone, they return to a norm that they now recognize as sub-optimal for being in the world. To this end, the person attempts to reassert that optimality in their pre-personal world, only to be confronted at each moment by the ending of that world. The realization of the end of their optimal world and the return to a sub-optimal world throws the individual back onto themselves painfully. We experience this as longing, grief, depression, pain, and it comes to characterize our world.
So, how do we get out of this? We don't. The simple truth of the matter is that, as organisms, we keep reaching for the optimal. But, the optimality that we experience can never be experienced again, that world has ended. It is not that we accept the end of the relationship and "move on," it is that we accept our new norm in light of the optimality that we once experience. It is the awareness of an optimal that keeps us reaching out into the world: once we know that there is something beyond our normal, that normal is no longer satisfying and we are driven to attain a state of optimality again.
To this end, we don't really "get over" something or someone until we attain a new optimal, until something enriches our world in a way that is different or beyond the optimal that we experienced previously. That is not to say that an intersubjective relation always provides this new optimality, but our reaching out into the world eventually grasps something that enriches our life in such a way as to make the optimal that we have lost seem diminished in comparison.
The new optimal is never the same as the previous, but it must exceed the previous optimal otherwise the organism will keep reaching out into the world for something, anything. Given the the affective allure of love, the way it throws us back onto ourselves to prompt a whole new way of taking up our own existence and transcendence, very few things can stand in for that intersubjective relation, and it is only in light of a new optimal that we are ever free form attempting to posit the world that we lost.