Well from what you are describing this sounds like a muscular pain more than anything else. The worsening of the pain with coughing and sneezing can be indicative of this problem. It is possible that you may have costochondritis which is a condition in which there is inflammation of the cartilage of teh ribs. this would be diagnosed on a clinical examination and then treated with anti inflammatory agents.
Now the two things which make me think otherwise are the absence of tenderness and the fact that you dont have pain on deep inspiration. These two features can also indicate that you have a pleural pathology. The pleura is the thin lining over the lungs which can be inflammed in a condition known as pleurisy. This would be evaluated on a clinical examination and also with a chest xray. This can be due to an infection and also due to other issues like autoimmune pathologies. So a chest xray should be done just in case here.
Another cause which should be excluded although it is the least likely one here is related to the heart. This should be considered in all left sided chest or breast pain. A simple EKG would exclude most conditions including pericarditis which can cause pain which worsens on coughing.
Now i think considering that you have been experiencing this pain for some time and it does not seem to be getting better, it would be advisable to have this evaluated by visiting your doctor. A simple clinical examination would rule out most of these causes but in the meantime start using OTC anti inflammatory agents like motrin or aleve which will help in some of the above conditions.
Do you have any more questions?
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