Although as it may sound very cheesy, but often those who would have gone through it, may be able to relate to this. What I speak here, is my take on my personal experiences and what I have seen around me.
To simply put it, love is an addiction, for the person you love, is your drug. When you are in a relationship, you tend to become each other's habits - be it physical, or psychological, or be it simply always wanting to wake up to someone’s presence around you. As one of my friend had once said, "The true success to a relationship is the mastery of adaptation." You adapt to each other’s way of living, each other’s habits; you schedule your daily routines so as to match each other’s schedules, so that you can (and you should) be able to spend the maximum amount of quality time. The factor of 'quality time together' is often the cause of trouble in the long distance relationships, yet many are capable enough to make the best out of it too.
Often things may not work out, be it for any reason or any kind of incompatibility; that’s when you decide to break-up. You may start feeling restricted, and metaphorically speaking, feel claustrophobic. So people part ways, sometimes even to a point of cutting out the other person completely. Hence, starts the period of emotional turmoils. Most are capable enough to handle this, but for those few, who feel the stress, why is that they feel so?
Imagine, a chain smoker or a heavy drinker, trying to quit their habits all of a sudden. The action is good, but the reaction doesn’t seem to be going right. The intensity of quitting may suit many, but not all. Their body rather starts reacting negatively, showing the signs of illness (or weakness), because their body had adapted to something which it isn’t receiving now. Yes, the habits are bad and would have caused harm, but the body doesn’t realize it and adapts to the unhealthy inputs it receives. But, all of a sudden when the inputs become nil, how is the body suppose to know and thus, starts behaving erratically.
In the similar fashion, when you think of cutting off the person would work, it infact doesn’t. You tend to fall in an infinite loop. And The loop goes as : Trying to forget a person -> Old time good memories creeping back -> Start missing the person -> Try again to forget (may even think about the bad parts of the relationship, just so as to give a reason to forget) and hence it continues. Conclusion : there is never a closure.
Now imagine if the things were taken slow, let it happen and be phased out in a gradual manner. Even the anti-drug for quitting unhealthy habits takes its time. Another example to think of is : when dams are getting over their overflow mark, the water is released, not all together, but in phases; because if all the excess water was released, the force of the water would be enough to destroy the neighboring lands.
Every part of your body is dedicated to its specific jobs and they keep doing it, even the heart; but your mind, its the only thing which has the power to adapt to the changing situations. You tend to mellow down while in relationship, and over the time the mind is trained to incorporate the other in your life. Post break-up, if there isn’t any time given to the mind to train and adapt to the fact that your partner is no longer going to be there for you, your mind keeps wandering in the loop. Your partner had become the input to your mind, but when it doesn’t find it any more, it keeps running in that loop, unable to find what it’s searching for.
This is the reason, why people keep falling in the memories of their past, because in their present their mind is unable to find the way into the future.