Believe me, my heart goes out to you! I understand how both you and your wife feel. My supported my husband through 6 years of depression. I am a nurse too and because I have experience in psychiatry, I am not sure if the Dr's listened to me more than my husband. My husband outwardly looked good, well dressed, well kept, but inside he was crumbling. I believe that there needs to be a balance between yourself, the Dr (maybe a referral to a psychiatrist - depending on the severity of her conditions) and the therapist. It can take a few different therapists before you find the right match. If you wife has suicidal thoughts, listen to them, hear her and make sure that the medical comminuty listens to her as well. That was my husbands' problem, I listened but the Dr's really didn't get how sick he truely was. I am sorry, but I am very passionate about this stuff, in May my husband took his life. He had seen his therapist that day (who had been good match), but there were no signs at that time. Mike decided that he was the cause of all of our problems and couldn't take the despair anymore.
If it has been 6 months that she has been on antidepressants without much effect then I think you should speak to your Dr about that. There are drugs that work better on some people, but 6 months is certainly a long time. Depression is exhausting and unrelenting. I don't know if I have any advice for you or not, but ensure that the medical professionals are listening to both your concerns and your wife's concerns aw well. What did help my husband when his depression was at its worst was having him get involved in a community group. He loved helping out and making a difference.
It is so good to hear that you are supporting your wife, that I believe is the most important key to over coming depression.