I was in this position until last year with my daughter and it was the most awful stress I have ever experienced - and I have had a very rollercoaster life. I swam everyday to relieve the tension and that helped a bit. While I was swimming I would face my fears head-on. Moving through the water made me brave enough to do this.
However, I would say, that the reason I was quite so taut with responsibility and pain is because my daughter was not taking responsibility for her problems and choices and using me to carry the load. And conversely, she was not having to face her choices or lack of choices because I was a barrier to any consequences.
In the end, through pure desperation, I did find boundaries. As a result, she started helping herself and is in a far better situation. In fact she has turned her life around completely. And our relationship is wonderful: so good to have fun with her.
I have bipolar disorder and it too me years to take on the management of my own disorder. It really was only when I became a parent that I stopped taking risks and started putting all my efforts into stability.
As a parent we can love and listen but bearing the load of another person's happiness or even their life is much too much; and, often, impossible. I am talking about near adults though. Obviously it is different if the child is very young.