Long distance for 18 months? That's like running backwards for 18 months. A few months can be achieved when you've only known them a short time, but 18 months is too long. Even if you tell them you'll try, the only way it's going to end is with you miserable or seeing other people then neglecting them.
Most people don't want to give up what they've got until they've got something better, so it's not like you wouldn't have company in that club.