Nobody can assure you of anything. As an American, you have a strong liklihood of being permitted to enter as a visitor.
However, it has been known for Americans wishing to enter as a visitor with a Canadian partner to be refused entry, and excluded from Canada (not allowed to come back for a period of time), if the officer felt their primary intent was to stay.
http://decisions.fct-cf.gc.ca/fc-cf/decisions/en/item/218891/index.do?r=AAAAAQAHVmlzaXRvcgE
There is no guarantee, as it is down to the officer's discretion. It is quite likely you will be fine. It is entirely possible you will not be. Until you get there and talk to an officer, you will not know. At the best end you'll be waved through in 10 seconds. At the worst, you'll be turned around and excluded. Anywhere between the two is possible.