I have, myself, several health issues (both mental and physical) and people keep coming to me with well-meaning advice that are, in fact, unwanted and totally useless.
They have a lot of theory about why I am the way I am and what I should do to get better. However, these people have no real understanding of how it is to be me, that things aren't as simple as they believe and that their advice are completely useless (because things aren't that simple) and that those advice are honestly annoying (no, doing yoga won't fix my issues).
My mother in particular would give a lot of such advice (how I should do more sport, eat healthier, sleep better, etc..) and would keep giving those exact same advice from one conversation to the next one. Here is usually how I deal with her:
If I'm feeling patient, I will try to explain things to her. And in those cases, it is very important to keep to the facts. It doesn't matter that I already told her this XX times. She tends to forget the details (everyone does. You are the only one who remembers those things perfectly because it's about yourself), that's why I need to keep repeating it to her and be as clear as possible.
Here are the facts A, B and C. This leads me to conclude X.
I present things that way so has to give as little room as possible for arguments. I don't really want to talk about this, so the shorter the conversation, the better.
In your case, you could say something like:
I already see a physician. I have had my depression for a year now and the flare for only 6 weeks. So, even if the two may be linked together, the flares can't be the primary cause of my depression.
If I'm not feeling patient, I would just say:
Is this what you wanted to discuss with me? Because I don't want to talk about it. So, if there is nothing else, I will put an end to this phone call/conversation.
Remember that this is your health and you don't have to talk about it with anyone if you don't want to.