1. What you are having (I would not use the term 'suffering from') is called 'benign nodularity'. This is a common problem having a hormonal
basis and there is nothing to worry about.
2. Now I will tell you the way to feel your own breasts to confirm benign nodularity. If you pick up the breast between the fingers, you will feel a big firm mass (even normally). And if you feel the breast with the flat surface of pulp of your fingers with gentle rotating movements, you will not feel that firm mass which you felt earlier; instead you will perceive a granular feel (similar to the feel of a gunny bag filled with grains like rice for example). The granular feel suggests benign nodularity.
3. Remember while feeling the breast with the flat surface of pulp of your fingers if you feel a firm or a hard mass (irrespective of its size), then it is a cause for concern and you must consult a surgeon at the earliest.
4. Regarding pain overlying the breast-bone, well, it has nothing to do with the breasts. Such pain is usually self-limiting though it may take some time (maybe months) to disappear. Try some simple symptomatic treatment like local application of a little Iodex (or any other similar ointment) followed by hot fomentation.
In short, at present, you have nothing to worry about.