How do I stop the constant pain & depression I have from being estranged (his choice) from my adult son?

I want to first acknowledge the pain and confusion that you must be feeling about the estrangement between you and your adult child and the fear that you must feel about the idea of never seeing them again as you grow in age. Losing a child no matter how old they are is difficult, and some of the most difficult losses in life are the losses of the living. It is immensely difficult to grieve the loss of someone who is still alive and this is really the task at hand. 

People cut off relationships with family members for a variety of reasons, some of them logical and others less so. Avoidance and cutting people out of their lives is something some people do when they don't know a better way to deal with their feelings. 

As a parent, I imagine you spend time trying to think about the things you could have done to cause the estrangement, but at the end of the day the decision to walk away from the relationship instead of work through complex emotions and repair the relationship was theirs. Unfortunately, adults who become estranged from their parents either don't realize the emotional impact of their actions or they lack empathy and emotional understanding of how you may feel. 

While you may have no control over the relationship, there are some things you can do to help soften the hurt. 

1. Explore ways to reach out without being intrusive. For example, writing letters that include updates on your life and photos and sharing favorite memories and that you think of them. Resist the urge to ask them to meet up or repair the relationship or ask about why they are not contacting you. While you may want answers, they may not be in a mental space to provide them. You can only open the door and keep it open, but it is up to them to want to step in. And they may not want to take that step.

2. Grieve the relationship. It's healthy and normal to be sad about the people you have lost in your life--even if they are still alive. Seek professional help if needed. Journal your feelings and seek support from friends and family. Remind yourself that despite whatever wrongs you did as a parent, you also did a lot of good things and you did the best you could at the time and are doing all you can now.