So anybody knows what’s the reason jaemin didnt confess early? Is it because daon is you...

Its cause he didn’t know how to express his feelings, he thought staying by his side was enough until others came into the equation then it became a problem, sometimes people don’t realize their mistakes until theres someone else.

It's late but here's my two cents. When they're in college, he planned to confess but that night Daon went to Soohyun's house and sleep with him. After sabotaging them, I think, he wants to wait until Daon completely gets over Soohyun.