I'm thankful for your concern for your wife, and I hope you'll do everything you can to help her. After all, if you were in her position I'm sure you'd look to her for help -- just as she's looking to you. The Bible says to husbands, "Each one of you... must love his wife as he loves himself" (Ephesians 5:33).
I'm not a psychologist or psychiatrist, of course, but your wife certainly seems to be suffering from depression (as her doctor says). Sometimes depression has a spiritual cause; King David, for example, was depressed after he secretly committed adultery -- and his depression didn't lift until he confessed his sin and sought God's forgiveness (see Psalm 32).
But often depression has other causes, such as biochemical imbalances in our bodies. Your doctor apparently suspects this is the case with your wife. Our brains are incredibly complex, and when something gets out of balance it can affect the way we think and look at the world. Often this can be corrected with proper medication.
Does this mean you would be leaving God out of the process, or looking to medicine instead of God for a solution? No, you shouldn't think this way. Instead, if the medicine works see it as God's way of answering your prayers -- and thank Him for it. In the meantime, constantly assure your wife of your love -- and of Christ's love, as well.