How do I get my mother removed as my power of attorney?

The provisions in the  Power of Attorney determine what your mother can or cannot do.  You can revoke the Power of Attorney by either following the stated revocation procedures (if any) in the Power of Attorney or you can revoke it orally or in writing.  It would be preferable to revoke it in writing so that there will be a record that you revoked it.  You can state in writing that you have revoked the Power of Attorney and give that written statement to your mother.  You could write something such as: I, _______ (your name) hereby revoke the Power of Attorney granted to _________ (your mother's name) effective_______(date).  You sign and date at the bottom.

As for the SSI, your mother doesn't have the authority to determine whether or not you continue with SSI.  Your eligibility to receive SSI is determined by Social Security or a court.  If you are able to work, you are no longer eligible for SSI.  Also, since you are young, Social Security may periodically require a medical exam to determine whether or not your medical condition has changed and whether you can work as a result of improvement in your medical condition.  After revoking the Power of Attorney, if you decide to cancel SSI, you can send a letter to the Social Security office cancelling it along with a copy of your letter of revocation of the Power of Attorney.