AFAIK you can't get your
TN visa until you are leaving Canada for the "last" time. That is the way it was for me, for my first TN.
Returning to Canada for a visit doesn't count. They don't worry about that -- but make sure you get your
TN I-94 stamped "MULTIPLE ENTRY" or when you return to Canada the departing airline may take it (they are supposed to take I-94s but they don't usually bother). And make sure they don't take it -- one airline tried to take mine once even though it was stamped multiple entry. It was a big argument I eventually lost, but fortunately I was getting a new TN on my return anyway.
Your intent to return to Canada is best proven by keeping property there, or at least an address where you stay when you are there (e.g. your parents). Unfortunately keeping property in Canada jeopardizes your non-resident status, which means you will have to pay Canadian
taxes after paying US taxes.