She occupies your every thought. You can't concentrate at work, and your friends are telling you that you seem spaced out. Thinking constantly about a girl to the point that it interferes with your day-to-day life is unhealthy, and you need to take steps to distract yourself and stop thinking about her. Train yourself to think about other things such as hobbies, work or other people.
Ask her out. If you're thinking about her all the time, chances are you like her. The only way to find out if she likes you too and put your mind at rest is to ask her on a date. Talk to her, call her or send her a message. Be direct -- ask her if she'd like to get a drink or a coffee with you, and then make specific plans.
Get busy. Find an activity so absorbing that it takes your mind off of her. Whether it's running, working on home improvement projects or studying Chinese, spend time doing something that challenges you and forces you to focus on something else. Make lots of plans with friends, who will likely change the subject when you won't stop bringing her up. The busier you are, the less you'll think of her.
Date someone else. If it's an ex that's preoccupying you, or someone who has turned down your offer of a date, the surest way to stop thinking about her is to find someone else to occupy your thoughts. Try an online dating site, or go somewhere frequented by women, like museums or wine tastings.
Change the subject. When you catch yourself thinking obsessively about her, literally change the subject of your thoughts. Replace thoughts of her with thoughts of anything else at all, whether it's world affairs or your grocery list.
Read more: How to Stop Thinking about A Girl | eHow