Exercise is good because it puts your body under acute stress which is what you're physiologically able to handle. A lot of psychological stress leaves the body under a chronic state of alarm which weakens the immune system and invites it to an increased chance of disease and depression. It's okay to think about the problem within relations to present, but becomes detrimental when you start to over think the issues. (Exercise can give confidence to a person that they can achieve goals, too)

It also has a natural "anti-depressant" effect for many.

I'm glad that you're finding a natural way of handling the situation.