Hmmm....ok, what we know biologically is that depression is caused by chemical imbalances particularly the serotonin, dopamine and (I always forget the other one). This can't be controlled with your power. That is why depression usually responds well to the appropiate medication.

Also, for example, if a person is HYPOthyroid, this causes depression. The low levels of thyroid hormones, particularly TSH and T4 cause problems in the binding of other chemicals such as serotonin. It is well documented that low thyroid levels cause depression. You can't control that...you need meds to control that.

Now, yes, you may be able to control situational issues. I have a crappy day at work, so I take a hot bath and relax and help myself get over it.
But you can not control depression or other biological illnesses by will-power.

Just my 2 cents.......