We all lose people in different ways. In ways we at times are not able to figure that what's the reason behind it. Why do we feel way?
The reason is that we tend to develop emotions, which is not wrong as it defines us as humans but they moment we lose someone or lose a connection our emotions spike futher. The reason behind this is that we keep on focusing on what we lost, instead of looking around. If you look around you'll see that if one person is absent or have left you there are a many people that are still with you. Example:
1. You lose a friendship and it affects your mood. You tend to isolate yourself and not give time to the friends who are still there or your family who deserves that attention. You take them for granted knowing that they go somewhere. So are these not the ones who deserve more attention and love? 
You wait for happiness to come back in your life forgetting the fact that it was with you the whole time and still is. All you need to do is put a little more effort to look around and change your perspective. Keep other people as your proiorty.
Work on yourself physically and mentally not for the sake of others fearing that they might leave you but for yourself. So that the next time you'll see yourself in the mirror you'll be impressed by the person you've become. 
In addition to this with people leaving you'll always have new people coming in your life, who will have a more healthy impact on you. There is always a balance but you never feel this way because you've closed all the doors for others. *At times it's alright to let people in