I'm am guessing that the puppy viewed the trainer as a pack leader because of her confidence and experience. He does not see you as a leader and is therefore challenging you.
This is going to take some time. I suggest getting Cesar Milans books or CDs on how to be the pack leader. Here is a link to some exercises that you can try, to help you to estalish yourself as the leader.
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This is an ongoing process and it will take time. Dont give up. As you become more confident the puppy wil begin to look at you differently.
It is good that you are concerned about this an not viewing as a phase or 'oh isnt he cute".
Good Luck