Congratulations on having undergone the breast reduction procedure; this operation tends to be one of the most patient pleasing operations we perform.The open wound problem that you are experiencing does occur occasionally after this procedure. Generally speaking, these areas go on to heal, within a few weeks (2-4) without long-term sequelae. Sometimes, if the scar is wider in this area, scar revision surgery may be helpful.
In the meantime, I would suggest that you continue to follow-up with your plastic surgeon. Occasionally, there is an offending suture that may need to be removed to allow for wound healing to complete its course.