Why does my mother constantly undermine me (her own daughter) but always overrates her niece (my cousin) and tries to compare her to me?

One of the main reasons I think your mother may be undermining you is because of her own inadequacy. and insecurity. I base this on my own adopted mother whom I never was able to live up to her standards. I was always being compared to her nieces or someone else and up to the day she died she still was undermining me. She felt if she could make me doubt myself that I would never be able to surpass her in life. The fact of her being my mother made no difference. Some of this undermining could have been something her parents did to her, and it was all she knew how to do. We are born in an unconscious state and we are unaware that we are in this state of sleep that keeps us below the veil of consciousness concerning the universe we live in. Awakening one's consciousness takes being able to live in a state of positive love and light that radiates from us and sends back what we send out. I guess like Karma. While living in that state of unawareness they can't do any more different than they do.

I have a few theories... 1. She sets unrealistic expectations for you and sometimes people often set unrealistic expectations that they can't meet themselves 2. She probably wishes that you could be her, she probably sees your cousin as perfect, because the expectations she set, your cousin is probably 'living up' to 3. She thinks (like many parents) that comparing your children to other children will "motivate" them and make them work harder. But again, this doesn't work and will only make someone feel bad These are only theories, I don't know the whole story, but based on what I do know, this is what it sounds like. Now only you can make decisions as it is your life. Hope this helped