History is All You Left Me by Adam Silvera

Griffin has lost his first love in a drowning accident. Theo was his best friend, his ex-boyfriend and the one he believed he would end up with. Now, reeling from grief and worsening OCD, Griffin turns to an unexpected person for help. Theo's new boyfriend.
But as their relationship becomes increasingly complicated, dangerous truths begin to surface. Griffin must make a choice: confront the past, or miss out on his future.