Generally, your mother must have capacity to revoke the POA. Alternatively, you can resign as the POA Agent. As long as there are no successor agents named, the POA becomes void.
You and your mother's doctors are in a difficult situation. If in fact she is well enough to handle her own financial affairs right now and revokes the POA or you resign your appointment, then you mother no longer has a valid POA, springing or otherwise. If she relapses and you need to take control of her financal affairs in the future, you will need to place her under a court ordered conservatorship which can be quite costly.
Check the spring POA to see if there is a clause that allows her to regain control of her affairs after she recovers without revoking it, but allows you to serve as her agent upon a relapse.
Your mother is presumed competent to manage her affairs, absent a decree establishing her lack of capacity or an order conserving her. Since there are no such orders existing in your summary of facts, she should be able to revoke the springing power she had signed earlier. However, because she lives in a skilled nursing facility, actions involving health care powers or business powers of attorney should be validated by the ombudsman who visits the residents of that facility. She should have the social worker at the care center schedule her in with the ombudsman at his next visit, who should be able to interview her and attest by his certificate, as to her ability to comprehend the meaning of her act of revocation.
I don't understand why she needs to revoke the springing power, since it is only triggered by the physician's certification of need as might be defined in the instrument itself. If that hasn't happened, than you aren't serving as attorney-in-fact. If it has happened, then you are serving. Does your mother distrust your service status? The fact that you are serving shouldn't necessarily be a concern to her, but if it is, she could revoke the instrument and either with the ombudsman's certification, or that plus her attending physician's certification, assuming that is available, she would be able to revoke it.
There are many elder care lawyers in the Santa Monica area who could meet with your mother and assist in this task; it is not a major project.
Best wishes to you and your mother.