Prior: I love you Louis. [...] I really do. But you can't come back. Not ever. (5.8.15-17)
Prior refuses to take Louis back; he can't forgive his betrayal. In the Epilogue, we see that Prior and Louis are still friends, even if they're not lovers, showing that their relationship has evolved into something new and hopefully better. It could be that Louis' abandonment of Prior ended up being good for both of them... maybe.