Letter to extend my tourist visa

Hi...

I would appreciate your help regarding extend my tourist visa. Now Im in Vancouver with my boyfiriend who is a Canadian citizen, I got my tourist visa from Mexico 5 months ago and it will expire in July. This is my sencond time that I got a single entry visa ( last year I came just for 1 week to visit and came back to Mexico to work) now I want to apply to extend my tourist visa since my boyfriend and I decided that will be easier to get married here and start the sponsorship process from within Canada so I can get my permanent status as spouse.

Im worry about how to explain on the letter that I got engaged with my boyfriend and we plan to get a civil marriage in the next few months. When I applied for my tourist visa this year I mentioned that I just want to come to visit a friend ( my relationship with my boyfriens now is more than 1 year). At that time I was working in Mexico and sent proof of my employment, however since I came here I quit my job and my boyfriend has been supporting me and also I have proof that I can support myself and dont need to pay for accomodation or food.

Would you think theres any problem if now I mention that I engaged?
What would you think is the best information I can add to support my letter?

Thanks for your advice...