Yikes. D: I'd tell her exactly how you feel. I'd say something like, "Mom, you really hurt my feelings by seeing that without me. You know how much I wanted to see it, and it's just not right how you gloat about going to see it without me. It makes me feel really crappy." It sounds corny, but if your mom is anything like mine (although mine would never do that to me in the first place), she should realize that she's upset her daughter and would do her best to remedy the situation. It's worth a shot, I think.
My mom WAS going to do that to me, no lie. We're big fans of.. Hugh Jackman and his assets, and we were talking about seeing Wolverine together before the movie even came out.
And then, I'm out of town and we're talking on the phone and she's all like "Oh yeah, Soandso's going to take me to see Wolverine." And then I hated her for a little while. Until I found out that she saw Star Trek instead XD
And then, I went and saw Wolverine with my older brother and sister, without her, and didn't have to pay for my ticket, so that was all in her face.
But, yeah, I totally know the pain of having a dating single mom. I always get blown off for the boyfriend.
edit: I'm not too butt-hurt over it, though. I mean - I'm an adult and I'm just very clingy to my mom XD It's weird when she blows me off, haha. I'm used to it being the other way around :D I guess it's payback?

I've already told my mom that, she told me to suck it up, lol.
But. Yeah. I'm not an adult, though, and I would never blow my mom off. ]: We're super close because for a major portion of our lives she was a single mother and I was her only kid, and now just ffffuuuuuuuu. I'm so bitter and jealous, it's disgusting...
Awww, that really sucks. I would be hurt, too. She should have been more sensitive to your feelings. Maybe you can go see it with a friend?
i guess she just wanted to be alone with her boyfriend. they might have not even gone to the movies, although i don't know why she would tell you that they were going to see "land of the lost" specifically in that case. but yeah, it makes sense that you are hurt. hopefully she just forgot that you wanted to go see that movie.