How do I focus when I'm around her?

I normaly have an amazing ability to focus, but now there's this girl whom i have feelings for. When I see her or sometimes even think of her I get lost in thought. It's getting to be a pretty big problem now, I've walked into a poll, forgotten to get milk (which I walked 2 miles to get), and almost been hit by a car because I was daydreaming about her. I know that I really like her and this is just how my body reacts to it, but it's getting to the point where it's harder to study because I start thinking of her. What can I do to stop this from happening? She isn't the first girl who I've liked, but when I think of her it just sends my heart aflutter. When I asked my friend about it he said "maby she's a psyren..." (it's a beast that hypnotizes men with it's voice and smile, lol). Why does this happen now? It was never like this before her. It's also annoing when I try to talk to her but all I can do is start blushing and make an excuse to run off. I think she likes me and we are both really shy.