There are several approaches to this.
1) You could change the topic of conversation to something that doesn’t remind you that he’s dating someone else. Movies often conjure up the image of dating. In contrast, sports, school, church, music, or funny YouTube videos can be more neutral conversation topics.
2) Decrease the number of interactions with him until you don’t feel that spark quite as strongly. Cut conversations short, politely avoid him or include others in your conversation so it's not a one-on-one conversation.
3) Turn your romantic attention toward someone else. Your crush is not the only attractive, special guy around. Even if you don’t like anyone else right now, challenge yourself to amp up your physical presence (attractiveness and energy), stand taller, smile, and look people in the eye. Talk to people you know whom you don’t ordinarily talk to. This will convey confidence and garner you positive attention. Your possibilities will begin to open up.
4) Confess and move on. He won’t be dating her forever. Therefore, make sure he knows you think he’s special so that when he’s single again, perhaps he’ll ask you out. Knowing that the information is out there (although he’s made another choice at the moment) can help you cope. DO NOT wait for him, but understand that you can be happy knowing you said what you needed to. You might do this by asking who he went to the movies with. Make sure the other girl’s name comes up, even if you have to mention her by asking if he’s still dating her or why he didn’t go with her instead. You can say she’s a lucky girl and look him in the eye when you say it. If you want to be brave and go further, you can say, “Can I admit something? I’ve had a crush on you, but I know you’re dating Sue, and I respect that. I’ve been struggling to get over my feelings. Let me just say she’s a lucky girl.”