Breaking up brings about a mix of feelings such as obsession for the lost one, hopelessness, fear, anger and loneliness, usually in two phases: 'protest' 'and 'resignation/despair'. Both these phases are driven by dopamine levels.
In the protest phase, the person might find it difficult to sleep and suffers from weight loss and nostalgia. All their time, energy and attention is spent on winning back the lost love (such motivation and focus is fuelled by dopamine). When the protest stage does not succeed anger sets in.
The next phase is resignation or giving up with a feeling of sadness and depression in some cases. This, at least in part, is due to dopamine, as it plummets it contributes to lethargy, sorrow and feeling of despair.
So it seems that all is related to the levels of dopamine.
One of my favourite L O V E songs, dedicated to dopamine, I guess :))))
Ehtiyaj by Shahyad
Lyric by Shahnam Farghar
عشق چیزی جزاعتیاد به دوپامین نیست! وقتی اینطوری نگاهش کنیم شاید به پوچیش بشه بهتر پی برد. اینم تقدیم به عشق که به عبارتی فقط یک انتقال دهنده عصبی یا همون دوپامین جان خودمونه. بودنش یه جور آزاره و نبودنش هم یه جور