How can it be? Well ... it's your life, and you let it be.
But maybe what you're asking is why do you let it be.
Could be a lot of things, but I'll hazard a guess that you know deep down that to confront this would result in a withering blast of contempt and to persist in confronting it will end the relationship, and you live in terror of being disliked and/or alone.
Your GF knows this and it gives her power over you. As a bonus, there's no need for her to improve her mean-spirited character; she can be openly petty and hurtful now that she's found someone with such low standards. And if she’s willing to model this behavior in front of her own child, I don’t think she is ever going to modify it for your benefit.
I’d recommend getting out of this relationship and learning who you are and what you want. Learn to love yourself, and learn to be completely happy and content outside of a relationship. THEN you could have hope of hooking up with someone of better character and stability, with fewer issues, because you wouldn’t be coming from a place of desperation – you could afford to be choosy, and you’d have something powerful to offer in a relationship.