Final answer:
The commas should be inserted around 'who I've been seeing since I was born'.
Step-by-step explanation:
The commas in the sentence 'My doctor, who I've been seeing since I was born, is going to retire' should be inserted around 'who I've been seeing since I was born'. The correct placement of commas is: 'My doctor, who I've been seeing since I was born, is going to retire.'