As an American, you shouldn't have a problem going to Canada to visit. I am also American and cross the border into Canada about once a week. I have never been pulled over going into Canada. All you need is a passport, which is simple enough to get if you don't already have one. You have plenty of time to get it. You don't need to tell them you are getting married when you go over. Just tell them you are going to visit your boyfriend, answer all of their questions honestly (they are trained to be able to tell if you are lying) and don't look too nervous. You aren't doing anything wrong by going to visit your boyfriend. You don't need a temporary residence visa, but from what people are saying on this forum, you can request one at the border. Americans do not need a visitor's visa to stay in Canada, and you are allowed to stay for up to 6 months. So, I believe that after 6 months, if you just left the country and went back to the US, and then reentered Canada, you'd be ok to stay for another 6 months. But I'm not sure about that.
I just wanted to tell you that the border people aren't really that scary, and you shouldn't be as worried as you sound.