Diet and exercise may have a slight impact but only if your condition was overall contouring of the breast due to mostly fat. This does not sound like what you are dealing with. Even if it does describe your condition the diet and exercise seldom makes a really big difference.
You'll find that there are a number of different compression garments available through sporting goods stores. These are very effective at concealing Gynecomastia. The do not make it go away, when you take the garment off it is still there.
If your condition is stable, and there has been no change for two years or more; Regression is not likely. If your condition has been this way for a shorter time, it is possible at your age that natural regression may occur.
Your condition is actually quite normal and for this reason, most medical insurance will not cover the cost of surgery.
If this is an issue that is really causing severe depression, your parents medical insurance may cover the cost of a Psychologist. A psychologist can be an important ally and in some cases they have been able to get insurance companies to cover surgery. What have you got to lose?