Hanbun Wa Ai De Dekitemasu
Complete | Sato Zakuri | 2008 released
Sadistic Boyfriend x Masochistic Girlfriend with a twist at the end. The twist is what made me search for this damn one-shot through hundreds of manga's, only using the words "boyfriend" or "sadistic" to try to find it. It left an impression on me so much I wanted to read it again but realized I never saved it to my Tags. Took some time... but I found it.
Soshite Koi ni Naru