I have seen similar issues of very late scar pain following by pass a few times. There is inevitably some pretty extensive disruption of the cutaneous nerves when the chest wall is opened. You end up with a lot of naked nerve endings,no longer attached to the normal skin receptors.When this happens nerves are only capable of transmitting one sensation when they are stimulated,pain.Which I am sure is happening in your case.
Trying to blanket cover over this pain with an opiate, or opiate analogue like fentanyl,would of course be a mistake. There is no point in flooding all your body systems in this way. You would need massive amounts to affect the pain,as fentanyl has little local effect,and you will inevitably get significant systemic side effects. Local lignocaine would reduce both the pain and the sensitivity,but would not solve the problem and might need to be used indefinitely.
On 2 occasions I injected steroid subcutaneously (Depomedrone with Lidocaine) which was reasonably effective in settling the problem down. Another possible option if this was unsuccessful would be a phenol nerve block to permanently destroy the nerves causing the pain. This would however increase the area of skin numbness, ( most patients have an area of this as well anyway)
GP for more years than I care to remember