>>90828101>Star falling towards Oskars hands
See mostly this: >>90828052
In the finale she has a breakdown in front of Moon that explains it. She 'guessed' she had to be with Oskar because he was her first when he arrived on Earth.
She also mentions that she chose to fix things with Marco, meaning putting up the farce that they can still be friends and not feel like shit. She honestly wanted to stay friends with Marco because they had amazing memories together.
Unfortunately however hard she tried to repress her crush on Marco, telling herself and everybody else that she can stay as friends with Marco and not be hurting AND telling herself that she still has a crush on Oskar... none of this was going to happen.
She honestly tried. But it wasn't going to happen. So the pressure made her finally confess.