Recognize that you're a stalker. It's one thing to pine over someone and pursue her, but it is another thing entirely to feel so consumed by this attraction that you do things that most people would consider unreasonable. If you are aggressively pursuing someone, yet the person is not responding or has even called you a stalker, then there is a good chance that your actions are bordering on stalking. You want to stop this before it goes too far and you wind up with legal action against you.
Tell the person you are stalking that you are going to leave her alone. This not only acts as a way to ease her mind, but as a definitive step for you toward ending the stalking behavior. Once you say you're going to stop, then make that your starting point of no more stalking. Realize that the other person is simply not intended to be with you, and there is someone else out there who will love you without any stalking necessary.
Find a replacement behavior. Stalking can take up a lot of time and you may find that once you stop stalking you aren't really sure what to do with your time. Try to find something productive to do that will keep you busy and won't allow you the time necessary in order to stalk the object of your affection. Let something else become your passion, whether it's a hobby, volunteering or a job.
Seek out professional help if necessary. Your stalking behavior may be indicative of a mental health issue that needs to be addressed, so make an appointment with a counselor or therapist in order to learn some coping mechanisms and get some prescription medication if necessary.
Forgive yourself for your behavior. Do not brand yourself as an obsessive stalker for the rest of your life, especially if you seek out the help you need in order to change your behavior. People can change, but it depends largely on their desire to change.