Idk if I got locked out from that CG because I unlocked all the event all at once. I tried giving deep talks, but still nothing tho. Thx anyways
If you get one ending, you can get the other by reducing the stat you achieved the first ending with through interactions until it's lower than the opposite or raising the latter higher than the former with positive interactions.
If you got the lust-based 'plaything' ending and want the happiness-based 'student' ending next, make her happiness higher than her lust before clicking the final unlock button again.
If you got the happiness-based 'student' ending and want the lust-based 'plaything' ending next, make her lust higher than her happiness before clicking it.
If you haven't altered her stats enough to change it, the game will notify you that the same sequence as last ending will play and ask if you want to continue.
Otherwise, it should give you the other ending.