It's a decision that you make with your head and not your heart. Making decisions with your heart can sometimes really mess you up. It will be difficult because feelings are strong. Months or even years from now you'll see how smart it was.
Time. Time heals all wounds. With time you will think less and less of this person. Before long you won't think about them every day, and then one day, you'll stop thinking about them entirely. If it was someone you love, you'll never forget them, but it won't hurt anymore. Good luck and keep your chin up.
Take in consideration all of viewpoints. For example, can you see yourself with this person years down the line? Is he/she stable (keep in mind the factors of mentality, drug-wise, job status etc.)? An important factor is also how they treat their family; it may reflect how they will treat you. All of these questions are things to consider before "following your heart". There is a reason our left hemisphere of our brain exists.;)
It's really rather difficult, you may love them forever. But your life paths are different, sometimes they will cross but you will always be on a different path. It is difficult to be with someone like this, you will always feel that little tug or strain and you know in your heart that true love is not this way and that true love is what you desire.