Steven Forth has characterized Canada's situation well.

I would add that with the NAFTA file as with most of the other files that President Trump has opened, he has generally used a "good cop, bad cop" routine blatantly; presumably, to keep those he's trying to pressure in a state of confusion. 

Presumably, the compromises that Canada made during the TPP negotiations are a given (provided that the US makes appropriate reciprocal concessions.  However, Canada must be prepared to refuse inordinate demands by the US for concessions even at the risk that negotiations break down and the US threatens to withdraw from NAFTA.

If we stand united, reasonable and firm, we can count on many interests in the US that will demand that the US, in turn, act reasonably.