It has happened to me every single time we need to split for whatever reason. She needs to travel and l cannot go, or l need to travel and she cannot come with me, it's all the same: at first l feel just fine, maybe even optimistic, but as days start passing l notice l have less energy, less wishes to do anything. I just start feeling weak or heavy. My life starts becomming a mess, then l start thinking about her more and more, dreaming about her, and the need becomes stronger.
Only once we had to be separated for several months, and l noticed l was depressed and started having suicidal thoughts. Nothing really serious, but l REALLY felt like crap and decided to seek a profesional as a precaution, while she was away.
It may be relevant to mention that l really love her a lot, in such a way that virtually anything she does or says, and even, for example, her shoes or whatever, seems cute to me. This has been exactly the same since the day l met her, about 8 years ago.
Now, l have this analogy to explain my feelings when she's away: it's as if l were living out of a batery. At first l don't feel anything, but after a few days l'm weaker and l function worst. Being with her is like my batery is full, and charging permanently, even if we discuss or whatever.
I read about certain birds that once they become a couple, they stay together forever, and if one dies, the other dies too a few days after. This happens sometimes among humans, but l wonder if there's a name for it.