@FJessymol Abraham... In the movie, Bai Qian said that she couldn't remember her teacher's face very well (it was 70 thousand years ago the last time she saw her teacher). Maybe that's why she doesn't know that Ye Hua and Mo Yuan has the same face. And it was also mentioned that Bai Qian became mortal because she wanted to help her teacher (the movie doesn't tell us why she becomes mortal or why she doesn't know that she is actually an immortal. Perhaps her memory was sealed). When she was mortal, she met Ye Hua. And when she jumped from the cliff, her (memory) seal was broken and she remembered that she was an immortal. But then she drank the water to forget her memory when she was mortal (and lived with Ye Hua). Ye Hua realized that Su Su and Qian Qian was the same person when Qian Qian made the same wish as Su Su (remember, Qian Qian is 140 thousand year old and he met Su Su 300 years ago. So, Qian Qian is not Su Su's reincarnation but they are the same person. Only something must has happened to Qian Qian 300 years ago that made her mortal). When Ye Hua died after the battle, I think his soul enters Mo Yuan's body & that's why when he woke up he called her Qian Qian.
Or maybe you can watch its TV series Eternal Love (aka Three Lives, Three Worlds, Ten Miles of Cherry Blossom). I haven't watched it, but I believe it will be more detailed than the movie.