I had open heart surgery a year ago. Now I have very little energy, arms go numb easiily and breathing trouble. Why is this?

Multiple things. Including rib dysfunction causing deep breathing difficulties, deconditioning from the procedure and the recovery, and possibly any medication changes afterward. The surgery can open the chest in ways not meant to happen and this can cause thoracic outlet like symptoms and other anatomical dysfunctions that can cause the numbness in your arms. Have CBC and bmp checked to make sure blood work is ok.