Depression can definitely kill a sex drive. If she really does mean as much to you as you seem to imply I think the best thing is to set sex aside right now. Instead, concentrate your efforts on helping her (and yourself) overcome depression. Figure out exactly what's wrong. See if the two of you can work through it. If not, bring in therapists. Once the depression is tackled her sex drive will hopefully return.
God, so atrocious in the Old Testament, so attractive in the New--the Jekyl and Hyde of sacred romance.
-Mark Twain
If people are good only because they fear punishment and hope for reward, then we are a sorry lot indeed.
-Albert Einstein