I’m sorry you are hurting and wishing for a different response from your mom. It could be that your mom is too self-centered to help you. But it’s also possible that the situation is more complicated than that.
Let’s look at another possible perspective: It does occur to me that just maybe sharing her problems is not dismissing yours. Maybe it is your mom’s way of relating to what you are going through. Or maybe she is trying to normalize what you are going through to something she understands. It also sounds like maybe your mom is overwhelmed and that maybe she feels helpless to help you. Maybe she accepts your older brother’s “help” because she doesn’t know what to do. All these are “maybes” because I don’t have enough information to get a sense of the whole picture.
Your mother’s problems don’t take away from your very legitimate concerns and disappointment. Adolescence is tough no matter what. It is especially tough if you don’t feel you have someone to turn to.
I strongly suggest that you talk to another adult you trust (a relative? a teacher?) about how to find a family counselor to help you and your mom. You have many years ahead where the two of you will be living together. With a little outside help from a counselor, the two of you can learn to understand each other. You mom can also learn new ways to be a better support to you.
I wish you well.