You probably cant, but Thea's things are about you and not him.
If he cares for you a great deal, he is going to be hurt. But, you can at least minimize the damage by being as honest as possible. Talk to him in person and tell him privately. Tell him that your feelings have changed. It is not fair of you to hold on to him when he could be out meeting his dream girl..the one that values him more than you do.
Change your relationship status on facebook to "single."