I am having numbness in the left back side of the chest especially while sitting for a long period and pain shoulder and I am diabetic.?

Many possibilities. Numbness can occur from various reasons. Some potential causes include vitamin deficiency, nerve damage, nerve compression, and alcohol over consumption, side effects of different medications, or as a result of different systemic medical conditions such as diabetes. Get evaluated by a professional to determine the diagnosis. Treatment is based on the diagnosis.

Pinched nerve. If this is limited to only the same area most of the time, then this may be due to a pinched nerve in the thoracic spine, please talk to your doctor.