Need advice please.

My gf is extremely depressed and has been all her life according to her. She is on Lexipro and still having thoughts of killing herself. She tells me she has thought in detail how she would do it, what music could be played at her funeral, and what clothes she would be wearing as well. She has recently moved into her own apartment after us living together for 3 years. I am having a hard time understanding how she feels because I have never had such feelings. I feel helpless and do not know what to do. Please help, I need some suggestions on what to do.

Well there is one distinction to make first about depression. 

Is it caused by a genetic, chemical imbalance or by her environment?  If she says she's been depressed all throughout her life... then it's likely the first one, but never make such a speculative conclusion without more research. 

What is her family like?  Are her parents divorced?  Is she an only child? Is she or does she think she is the 'ugly duckling' child?  Has anything happened recently that might affect her mood?  Fail college? Get pregnant? (might want to make double sure on this one) Lose a job? Fail at a life goal?

If you can find any evidence of these, then there may be something you can do to help her, but its going to be selfless and altruistic as hell and you shouldn't rely on receiving any encouragement or positive response from her, depressed people are rarely appreciative of attempts to help.  They feel that depression is inevitable.  Just try to think of things to get her to do.  Things she'll enjoy, but more importantly new things.  Depression is often accompanied by the feeling of 'being in a rut', difficult to break out of for a person on their own.

You may also try to make her feel needed by you, this may be effective if she feels purposeless. If she gives the impression or speaks as if she isn't necessary to the world, try to work this in.  DO NOT go overboard on this, keep it to the barest minimum, ask her to help you do something because you think she can help you or because you think she is better at it than you.  Be patient, likely she will not be enthusiastic or will not respond at all, just keep at it, but give her time between your requests, be subtle if you can.

Adrenaline, its addicting sure, but its also something your body is designed to encounter fluctuating levels of constantly.  It also does a pretty good job of flushing depressing thoughts from your mind.  Think of something exciting and surprise her with it, drag her out if you have to (unless of course she is deathly afraid of it). If the activity is naturally exciting or even frightening it will have the desired effect, though again.. she may or may not be appreciative.

If none of these apply, nor anything similar then it's likely genetically low serotonin production and the above efforts may have some effect, but any positive effects will be mitigated by chemical imbalance.  Lexapro is already one of the more powerful anti-depressants and if that isn't helping, it's unlikely that anything else will. (Although there is a possibility that she reacts abnormally to it, a very small possibility.)  This is when it's appropriate to see a psychiatrist, when everything else has failed.