How do you feel when you/SO leaves?

I've been in a LDR for almost 16 months now. I have depression and anxiety, for which I'm on medication and going through all of that to try and find something that helps. I just wanted to know how you feel when you leave or your SO leaves to go back to your/their home? I feel like it's the end of the world and I have no purpose anymore and like I'm dying and it feels like my SO has died and it's horrible messaging on WhatsApp because it's like a robot has taken his place... I don't really think any of that is normal and I think my mental health makes it worse but I just wanted someone to express their feelings too?