Here is the full list of all 69 Returning To Mia achievements.
Unlocked the gallery
Chose Mia forever
Didn't turn Isla down
Went inside with Sadie
Didn't go inside with Sadie
Got slapped, hard
Got rid of the towel
Had to make a "difficult" choice
Failed to dodge properly
Trained with Mia
Agreed to come to Giselle's office
Lifted an EXTREMELY heavy box
Picked Mia over Sadie
Told Cindy you are interested
Helped Grace out
Chose not to stay with Mia
Chose to stay with Mia
Accepted Simon's proposal
Spied on Ellie and Vivian
Talked to Pete for the first time
Admitted you have fantasies to Sadie
Used the "Two in one" button
Complimented Mia's costume
Told Cindy the truth
Kissed Isla
Had to change pants
Took Sebastian's pants
Remembered your past with Mia
Relaxed AGAIN!
Became a "Spartan"
Met someone familiar
Thought about Vivian
Thought about Cindy
Thought about Grace
Thought about Mia
Thought about Isla
Went looking for Sadie
Talked to Giselle for the first time
Talked to Sadie for the first time after a while
You relaxed!
Went to dinner with Grace
Got to know Cindy better
Found out Mia want you to stay
Got rid of a bully
Found out Cindy is Sebastian's daughter
Talked to Sebastian for the first time
Talked to Cindy for the first time
Used the "Angle button" once
Talked to Mia for the first time after two years
Talked to Grace for the first time
Used a certain little red button
Talked to Vivian for the first time
Started the story
Used the "Panic button" once
Flashback!
Talked to Isla for the first time
Gave good advice to Isla
Used the back door
Refused Simon's proposal
Picked Sadie over Mia
Found a mysterious note
Rushed to Sadie in a time of need
Turned Isla down
Talked to Hope for the first time
Reunited with Sadie
Proposed to Sadie
Ended up single
Found all four endings
Collected all other achievements