What have the doctors told you? Is there nothing wrong that they can find? Stress can really cause a lot of problems for men but so can a poor diet, poor health and being out of shape.
You can start by eating a healthy diet, if you don't already. Avoid highly processed foods. Try getting your whole body in a better physical condition. Start some kind of exercise that you enjoy. You can also start exercises for your sex muscles which target the muscles that control your penis and erections. Exercising these muscles will make them harder and stronger and give you more control.
If you have a lot of stress try finding ways to release it. Exercise in general usually helps. Things like meditation, yoga, and tai chi are also very good.
