This Sweet Sickness(1977)

Original title: Dites-lui que je l'aime

Industrious accountant David Martinaud claims to be visiting his parents every weekend, but it's merely a cover for the labor-intensive refurbishing of his country cottage. The reason for David's secrecy is that his childhood crush, Lise, lives nearby, and, although she's married, he still believes they'll get together. His devotion to the idea borders on mania, and, when Lise's husband is accidentally killed, he quickly spirals out of control.