Hey,
A colleague of mine had depression through work from the office, and he leaned on me talk about it. Its a terrible thing to have.. and i imagine you feel kind of helpless becuase there's nothing you can say or do take this problem away from her. So my advice to you would to sit tight, just be there for her, offer her lots of reassurnace, and when the s*it really hits the fan, all you have to do is stick your arms round her and tell her that everything is gonna be alright. Its the greatest comfort of all.
In terms of you being a sucker for this.. i don't think anyone can really comment on that, as long as your happy, and you can handle the situation, which at times i garauntee will be tricky, i think you'll be fine. Just keep her talking to you about it, and let her lean on you when she needs to. The other thing i'd say is to try and be as "normal" as possible with her, keep taking her out places, and meeting with friends etc... , it'll take her mind off it, and maybe speed up the recovery process, don't let the illness affect every part of her life. If its anything like my experience there'll be periods of awkward silence when she's not talking and you feel that theres nothing left to say that you haven't already said, so maybe something else to adjust to.
In the event that you can't handle it anymore... she's likely to come back at you with allsorts.. and i imagine you'll be left feeling pretty guilty about leaving her when she's in this position, but don't forget, your not bound into this relationship, its ok for you to end this if it gets too heavy.
As for the sex drive...its probably not on her "to do" list if she's not feeling too good about herself, so just be patient, it'll slowly get back to how it was. Sometimes being intimate with someone doesn't mean just sex.
I was once told "fall down seven times stand up 8." Maybe try keeping her in a positive frame of mind, although for her it may seem like the end of the world!!!
So just lots of reassurance that you're there for her, and that you really do care... can't stress that enough. Just because she feels down- doesn't mean she has to stay there.
Hope this helps