It hurts. Maybe everyone will say you deserve better, and maybe you'll feel otherwise, because of the great emotional attachments to him. It's okay to feel that way, and it's okay to have feelings. Let it hurt, in my opinion that would be the first thing to do.

Then screw him! And focus on your life. At first it will be difficult, because he's all around you. TRUST ME, I GET THAT. But eventually it WILL get easier, and you will get tired of it. This will NOT last forever. Change jobs if you'd like (but I guess it will be difficult since you need to put your students' needs ahead of you), your emotional health and happiness matters. And you probably won't be able to do your job well if you're affected, right? But stay strong. If you need to stay in the same school, stay strong, and it's not like you'll see him 24/7. On the moments you do not see him, make use and treasure that alone time to heal.

Seeing a therapist sounds like a good idea. You need time for healing. You've got a lot on your plate. Get angry, get upset, rant all you want. You're all that matters now, not him. He's the one who made a serious mistake.