It seems that your mother is in denial about what happened. Such a psychological defense mechanism can be hard to change. Your mother may need time on this. If she sees your support and unconditional love, then this may help reduce the denial. When she sees and experiences your unconditional love try—gently—bringing up one concrete instance of neglect in the spirit of forgiving. The concrete referent and the unconditional love in combination may aid your mother in breaking the denial and being open to your forgiveness of her.
For additional information, see My Mother Robbed Me of Trust.