Complex sentence: In the end, Nat out that Rosalina thought he was in love Grace and say "No, I love Rosalina, how could she not know I love her" but Nat didn't know she was in the room.
Split into simpler sentences: In the end, Nat found out that Rosalina thought he was in love Grace. He then said "No, I love Rosalina, how could she not know I love her" but Nat didn't know she was in the room.