>Also you learnt an interesting thing: her boyfriend isn't willing to pay to spend some good time with his girlfriend in an amusement park
OP didn't mention that he knew the reason the boyfriend couldn't make it. It could be that he had a previous family commitment.
While I agree with what you're saying about not being a lapdog I think you have to be careful in regards to how important her dissapointment is:
>Seconds, she will be disappointed that you can't go. "disappointed" + "you not there" = win
She's going to an amusement park, and I doubt she's going on her own. It's hard to stay disapointed at an amusement park, those places are fun. Hopefully there will be some element of the OP being in the back of her mind, but we don't really know enough about who was going with the girl etc etc
All in all though, good advice - especially about trying to subtly avoid talking about the boyfriend and focusing on having lots of fun together - OP you should definately take that away with you