I have been seeing my obgyn for 3 months now. I had an addiction when I found out I was pregnant and told him about it. He started prescribing me methadone. Well I am switching doctors and can't quit my methadone cold turkey. How do I go about explaining this to a new obgyn

You don't - your current doctor explains it to the new doctor, and provides your other medical records. Some part of this story is missing? I don't understand why this is even a problem.

You simply tell them you are on a methadone program as soon as you see them.

Be sure to let your current OB know that you are switching so that your records will be forwarded over. Your new doctor will then be aware of your medical history and will discuss it with you. Do not change your current dosage... your new doctor will monitor you and baby for the remainder of your pregnancy and make any adjustments as needed.