The probation officer said you could lose your deferred status, not that you will lose it - that's not all bad. It may be that the probation officer is giving you a chance to redeem yourself.
Telling the judge "I didn't think I would be tested" is almost like saying "But I thought I was going to get away with it". It would likely be far more beneficial to be sincere and apologetic, and take responsibility for your actions.
You may wish to consult a local criminal defense lawyer about your situation.

