OP, I agree that you need mental health help quite seriously.
You've had 18 years with a much-loved child, who is now going and exploring the world. He may well have children of his own in the next two decades. This can be a joy and a celebration.
And family and love isn't limited by direct birth or even by blood. If you want, your great-nephew or great-niece can become another beloved family member, who will in turn love you and think of you warmly and fondly. Same for your friends' children and grandchildren. We can build villages and forge strong bonds with all sorts of people.
That you don't feel able to do this, after twenty years, suggests that your anguish over not having a second child is a symptom of a larger problem, not the problem itself. (If you had had two, perhaps you would have felt like this about not having a third.)
All of which is to say, you sound like you are in serious pain, that really has very little to do with the actual situation, and you should get help for it as soon as you can.