How do I get over him?

I’ve been finding it difficult to fully get over my ex boyfriend. We are still good friends, considering we were best friends before we dated. I thought I’d be okay with being friends, but it’s bringing back old memories whenever we talk. The thing is, we have a class together and we are assigned to sit together. I’ve already tried changing seats but I can’t. I don’t know what to do anymore! I feel like since I have to sit with him it will just prolong the healing process. How can I prevent that? Please help!