You were an engineer for the Solaris space colony on jungle planet Xerxes beta, you were out on a mission to rescue someone and on your way back, your oxygen started failing, you almost died from carbon dioxide poisoning right outside of the ship you were meant to take the person you were rescuing to. The person died but you lived just barely. You come back to consciousness in a hospital bed surrounded by doctors, back on the colony and safe. But the trauma is still there.