Evergreen by Cari Z.

Soldiers. Explorers. Lovers.
Broken apart.
Cy Konstantin and Scottie Andrews are supposed to make Project Evergreen’s one-way trip to Mars together. A near-fatal accident during training knocks Cy into a coma for half a year, and out of Project Evergreen. He works his way back to Scottie’s side, but he can’t rejoin the mission. Once Scottie leaves, they’re destined to live millions of miles apart for the rest of their lives.
A deadly accident on Mars might spell the end of their distant romance, though—or be the thing that saves it.