Golly, this is going to sound like a dull answer, but I think you need to sit her down and have a heart-to-heart with her. Take a deep breath and tell her, "Gertrude, you know I love you, but your constant negativity and self-deprecating comments are really wearing me down. It's getting to the point where I am reluctant to spend time with you, because no amount of reassurance on my part seems to help your self-image. Is there anything concrete I can do to help you develop some self-esteem? If not, I fear our friendship might be in jeopardy, despite the fact that you are a beautiful, attractive, funny, kind person whose company I truly enjoy. I just won't tolerate listening to you badmouth someone I love that way."..

I might say to her ... "Betty u know you really have to stop putting yourself down, ever hear the saying, "if someone tells u something about themselves ... believe it"... You really have to curb that negativity."..
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I'm a believer in positive energy and the effects it has on the mind, body and soul...Ever notice how positive people, attitudes, outlooks really do attract positive results...
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Angie..
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I'm the yin to Carlos' yang .....

Not only do you need to tell her the effect it's having on you, i.e. not wanting to be around her sad-sack (but sweet) arse, but try to show her the negative effect it must be having on her. If she's always thinking "oh poor me" then it will be a self-fulfilling prophecy and she will be poor and unattractive and pathetic. No one ever got anywhere thinking like that. She needs a complete rehaul of her attitude and outlook, which may require talking to a therapist or may just require an occasional kick in the ass, whether it's from you or from herself...