My fiance had a depressive episode this past summer. I was on the phone with his therapist and psychiatrist often. It's not your fault. Nothing you did made this depression rear it's ugly head. I repeat, it's not your fault. You're a team and you both want her to get better. Sometimes (times like this, when she cannot fight for herself), you need to take her health into your hands. It certainly sounds like she needs a med-tweak. Does she have a reliable psychiatrist? Could you call him/her? Does the therapist have permission to talk to her pdoc? Does she have insurance?
I KNOW the pain you are feeling all too well right now and it makes me emotional just bringing my thoughts back to that time. It will get better. Even when I didn't believe it myslef, I still told my fiance every day that it won't always be like this and he will get better. If you are committed to her and believe that she'll heed the advice, med-change, perhaps even hospitalization, then I strongly suggest you take matters into your own hands and get on the phone with her doctors. Does she have family who are loving, understanding, and supportive of your situation? If so, reach out to them, too.
Her lamictal dosage can be adjusted, the pdoc can add an "activator" anti-depressant to lift her out of this, or her mood stabilizer can be adjusted (although I'm not a dr, and what i just wrote was speculation...i just want to reiterate there is hope).
Keep us posted.