Dear (insert your name here),
I have been doing some (thinking, research, math), and it seems that I was wrong about (our relationship, your test results, your past-due account). It turns out that (I still love you, your health is fine, you don't owe us any money) and (I was crazy to ever leave you, the data was off, the bill was miscalculated).
Please accept my (apology, apology, apology) for any (heartache, worry, trouble) this has caused you. I hope that we can (get together and talk about it, satisfy your healthcare needs, continue to do business) in the future.
Best wishes,
(Everyone Who Broke Your Heart, Doctor Whomever, Creditor X)