So at first Lusi returned/back to the past through the bodies of other person whose faces were similar to her and then she met Song Weilong and then they fell in love
(i.e. Song Weilong in the past and Lusi in the future fell in love)
and when Lusi returned to the past for the second time they decided to married but when they getting married
actually she's not the real Lusi anymore but someone who looked like Lusi, that's why she said "You can love someone but the person you fall in love with is not me bla bla...."
(Because the girl that Song Weilong loves is Lusi who is in the future who at that time she's in the body of someone who looks like Lusi in the past)
then she (someone who looks like Lusi) committed suicide,
and at the end of the scene, Song Weilong from the past strangely into the future and enters the body of someone who looks like him and uses that opportunity to find the real Lusi and they meet again when the plane crash hasn't happened yet...