All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
First of all, we thank you for being keen to be kind to your mother. We ask Allaah to bless you with the good of this world and that of the Hereafter for doing so, and we ask Him to bless you with a pious husband.
There is no doubt that the children are religiously ordered to obey their parents, but this obedience should be within what is permissible. However, it is not within what is permissible for parents to ask their daughter to marry a man who has certain characteristics, and if such a man is available she will marry otherwise no, as this may cause clear harm to her.
If this is confirmed, then it is permissible for you to go against the will of your mother and marry a suitable and competent man who proposes to you, even if your mother is mentally sound, let alone when she is a schizophrenia patient, as you mentioned.
Finally, it should be mentioned that it is permissible for a woman to look for a person to marry her provided she observes the religious requirements in doing so. For more benefit, please refer to Fatwa 83870.