What do you do if you give a girl a valentines card and she already has a boyfriend?

I made the mistake of giving a girl that I like a valentines card because I thought she was bluffing about having a boyfriend on her facebook. Well, it comes to find out that she really does because I saw his name written all over her folders. I gave her the card and she didn't even say thank you. I felt very bad afterward. I didn't know what to say or do. What should I do now? She acts like she likes me even though she has a boyfriend. That is why I thought she was bluffing.