What about her do you like, why are your her friend/boyfriend? I would look to your own interests, what you want that's positive and where you see that in her, I would compliment it. If she is "funny" and you enjoy that, be warmer and more engaging at that time. If she is complaining and you don't like that, tell her; "Hey, I don't enjoy listening to this, can we change the subject" (or say you have to go and leave, go do something else you enjoy more with someone friendly and appreciative).

Her moods are not your problem. She has to learn to respond to others in a way that gets her what she wants in life; we all do. You can help by modeling behavior that shows what you want and how you are in the world and what you want when you are with her. Bitterness is a form of paying more attention to the past than to the present. the more "present" you are and attractive to her, you can help her come into the present with you. If she persists in looking at what she doesn't have, what bad things have happened to her in the past, etc. then you take the present of you :-) away. She could learn that she prefers you and your present to her bitterness.