Hi there! I can appreciate your dilemma. Although anyone in the health field is capable of making a mistake, I can say without reservation, that your psychiatrist is the person to listen to. There are plenty of reasons why psychologists can't write prescriptions, first and foremost, they didn't go to medical school. They don't receive extensive training as diagnosticians.
They get their bachelors degree, and their PhD work is often in some narrow part of the psych world. Compare that to an MD with 4 yrs pred-med, 4 years medical school, 1 year internship, and another 3 years of a rugged residency in psychiatry, which prepares them to take on this difficult specialty. I'm sure your therapist/psychologist was well-intentioned, but if they even diagnose you with great assurance or certainty, they are over-stepping professional boundaries. They can make their best guess, but they aren't qualified as diagnosticians, period.
A lot of MDs I know would just dismiss their Dx, and even find it insulting. If you feel you are bipolar and not MDD, I suggest you present your bipolar symptoms to your physician again, along with your ADD symptoms, and ask very directly why he has Dx'd you as MDD, and not Bipolar II. I hope he will clear things up for you, he owes you that answer. If he is unwilling to answer such an important question, certainly of great importance to you, then it might be time to shop around for a doctor with whom you have better chemistry. **Never let a PhD diagnose you and call it the final word. Good luck to you.