It is time to deal more directly with your mother’s emotional neediness, manipulation, and unrealistic thinking. Abandoning your own life to help a mother unwilling to help herself adequately will continually drain you. There is no delicate end to this type of situation. Once your brother turns 18, your mother will simply find other ways to manipulate you into staying.
You mother’s anger is the thing that is causing your guilt. The work is to get yourself some support through individual or group therapy and make a plan to get out from under your mother’s control. Your mother is robbing you of your life in order to feed her own. This is a selfish and irresponsible way of parenting.
I’d encourage you to make a plan to move on with your life, while helping your mom get services she needs. Ideally you and she should go to therapy together so the therapist can help broker this process — but if she is unwilling I’d make the appointment for yourself and get some direct guidance as to how to reclaim your own life.