You likely cannot change your mind now. When you choose a mitigation hearing and choose to go forward with the hearing, you've admitted that you committed the infraction. If you go in and ask that your deferral be revoked, the judge will probably just find you committed.
Its probably too late to go back to court and tell the Judge that you changed your mind about the deferred finding. However, you may be able to file what is called a "Good Cause" motion. This tells the Judge of some good reasons why to vacate the deferred finding and set a contested hearing. I believe you would have a better chance of success with such a motion if you hired an attorney.