Yes, competing with US sellers IS difficult, and as Monique says in another thread, most of us learn to work around it.
In my situation, for example, an average book weighing .75kg sent to the US via Canada Post costs CAD$12.75/Small Packet Air or $10.30 Small Packet Ground. The same book can be mailed in the US via Media mail for US$2.07 or flat-rate Priority service for US$4.06.
I pay a courier to take my books to Niagara, and then pay an agent to take them across the border and mail them from the US, and still can charge my customers less than half what it would cost to ship from Canada. That is truly unreal when you think about it.
Ann