Hey, tbh i've never heard of anyone only taking the pill around the time they are seeing their boyfriends. It seems a bit strange to me, what's the problem with taking it all the time? Personally i doubt i'd know if i was coming or going if my hormone levels were taking a battering like that!
Anyways, aside from that i'd thought i'd give you some reassurance that your lack of periods is probably not an issue - i've been on it for almost a year and haven't had a single period in that time, and i'm definately not pregnant! Apparently it's a common side-effect of Cerazette that periods stop - i'm not sure why, but personally i think it's a blessing not to have to deal with all that mess once a month.
Not really sure about the weight thing, but i weigh about the same as you and have never been told that i was too heavy to be on this pill.
Not sure i've really answered your questions, but hopefully i've given a bit of reassurance.