How do you break up with someone that's not as great as you? It's easy. Just keep in mind two things: No one out there is as great as you, and great partners are easy to come by.
Let's tackle them one by one.
No one is as great as you. If they were, they would be with your girlfriend/boyfriend right now. But they're not, so now you're girlfriend/boyfriend is sure to live a life knowing they've settled for less, romantically. Isn't that great? Doesn't it feel good to know that for somewhere out there, whoever they are with is only because they're not still with you? Power. It gives one a great sense of power.
Nothing is easier to come by than a great partner. If you are in a lawn and garden section, and you're looking at lawn chairs stacked 12 feet in the air, priced 3 for a dollar, remember that truly great partners are even easier to come by than that. Ask yourself these two simple questions - Are they desperate/capable of desperation? and Have they already rejected me? You want a "Yes/No". A "Yes/Yes" will also do. A "No/Yes" is the hardest, but it can work as long as you constantly remind them how great you are.