There is great advice in both answers above. This is a kind of concrete answer to fill in the gaps.
This has been said already, but bears repeating: Therapy. Therapists are not all created equally. Get a good one and if there doesn't seem to be a good connection between the therapist and your gf, offer to help her find another.
Self-compassion is different from self esteem. Her low self esteem is reinforced by the negative self-talk that she's incorporated, and it's difficult to argue with those voices. While she's working on the negative messages of her inner dialogue ("I'm worthless"), encourage her to think of herself as deserving of compassion. It's pretty hard to deny that one is worthy of compassion, and easier to tackle than self esteem.
Results from [five studies] showed that self-compassion predicted emotional and cognitive reactions to negative events in everyday life, ...buffered people against negative self-feelings when imagining distressing social events, ...moderated negative emotions after receiving ambivalent feedback, particularly for participants who were low in self-esteem. ...self-compassion leads people to acknowledge their role in negative events without feeling overwhelmed with negative emotions. In general, these studies suggest that self-compassion attenuates people's reactions to negative events in ways that are distinct from and, in some cases, more beneficial than self-esteem.
Self-compassion is a kind of non-judgementalism, of treating oneself as one treats others.
Self-compassion entails three main components: (a) self-kindness—being kind and understanding toward oneself in instances of pain or failure rather than being harshly self-critical, (b) common humanity—perceiving one's experiences as part of the larger human experience rather than seeing them as separating and isolating, and (c) mindfulness — holding painful thoughts and feelings in balanced awareness rather than over-identifying with them. Self-compassion is an emotionally positive self-attitude that should protect against the negative consequences of self-judgment, isolation, and rumination (such as depression).
One way to start practicing self-compassion is to journal. Journaling will reveal some of the negative self-talk that reinforces low self-esteem. When she later reads what she has journaled, she can pick out the negatives and try to evaluate their "validity" as if someone else in her shoes were speaking. Would she agree with their assessment? If yes, why; if no, why not? Would she have compassion for that person? How would she advise that compassion be manifested?
You can't be her therapist, but you can help her to have compassion on herself, not because she is "a good person" (she doesn't feel like one), but because she deserves as much compassion as does any other human being trying to live. Then help her to find a compassionate response when she expresses negativity towards herself.
One of the most important aspects of this is that she not judge herself for doing this "wrong" or "badly". Don't tell her she's wrong to feel the way she feels. Facts can be wrong ("You aren't worthless/You are kind/you are intelligent"; those are facts.) Feelings aren't facts; they aren't right or wrong. They just are.
If you need concrete help, I'd advise a journaling guide called The Self-Esteem Guided Journal: A 10-Week Program (New Harbinger Guided Journal). Though it is labeled "self-esteem", I would put it in the self-compassion category.
Another powerful resource is a book called "Wherever You Go, There You Are: Mindfulness Meditation in Everyday Life" by Jon Kabat-Zinn. It teaches how to practice mindfulness and gratitude, both important to feeling better about life. Zinn is no fly-by-night psychologist; he has a PhD from MIT in Molecular Biology, but has made stress reduction his life's work.
You can't fix your girlfriend, but you can encourage her to treat herself with the same kindness with which she treats others. That's incredibly powerful.
Self-compassion and reactions to unpleasant self-relevant events: The implications of treating oneself kindly.
Self-Compassion: An Alternative Conceptualization of a Healthy Attitude Toward Oneself
10 Journaling Tips to Help You Heal, Grow and Thrive