This is an open-ended story, which means you as a reader ultimately decides whether it's a happy, sad or bittersweet ending. I did think about just going for an established ending but after numerous revisions, it seemed rather dull--at least compared to the open-ended conclusion in the finalized shot where a little bit more reader imagination and involvement is forcibly induced.

I suppose my need to deviate from the norm is also to blame; where most works present the "This is the ending; what do you feel?" conclusion, I wanted to go for the "What do you feel is the ending?" conclusion.

• The happy ending: It was all a dream; plain and simple. Everything that happened in the beginning was just a lucid dream to Yoona and that voice she heard in the end was a recollection of what the older Seohyun from her dreams said as Yoona was still waking up. You know how dreams stick to you when you're in that groggy half-asleep, half-awake state? And that statement in the end was just an expression on how good she has it with Seohyun (comparing her reality to heaven).

• The second possible ending is a bit more complicated/versatile; everything that happened in the beginning was real. Perhaps when Yoona left the older Seohyun she went into some sort of plane void of time and space; a place where dead people go; kind of like some form of heaven or afterlife zone where you recall your most happiest memories, relive it and then sleep until reincarnation. Or, since it cut off early, it might be that she'll 'wake' up and continue 'living' in that memory eternally rather than 'sleeping', but happy all the while.To Yoona, her happiest was when she was younger and deeper in love with Seohyun so the scene in the end in a sense is the dream. When she heard Seohyun's voice--the real Seohyun-- she realized it then that she wasn't really alive, that she was in some sort of blissful afterlife hence her statement in the end, "So this is what heaven feels like...". Yoona embraced it in the end knowing she'll be happy but ultimately, that Seohyun--the real one--will be happy as well especially without Yoona holding her back from moving on (possibly with Yuri).

As a reader of my own story, I chose to believe that the second option is the actual ending although bitter-sweet since it leaves more room for reader imagination and interpretation. I've had one reader merge the two endings together where Yoona is actually still alive, although in a hospital lying in a comatose state as she dreams. Also, I refuse to believe I wrote all that just for a dream lmao.

A different reader pretty much summarized my thoughts: 

"It wasn't a happy ending for me because though Yoona is reliving her life with Seohyun in another world, Seohyun still bears the pain of having lost someone who encompassed so much of her life. Even if Yuri and Seohyun supposedly get together, it's not the same because it's kind of like what Seohyun joked about "Because she looks like you. In the end, I’m still thinking about you"...you get the feeling that Seohyun never moves on. How she saved the last message where Yoona says three important things, "I can't live without you", "I love you so much" and finally "I'm sorry" and the way she kept comparing Yuri to Yoona then her final words to Yoona before she "officially" left "Please, don't leave me...". In Yoona's case, she can't help but move on. And you find yourself feeling equally as sorry for her because she's living a fantasy. When you added the part where Yoona hears Seohyun pleading, one gets the feeling that subconsciously Yoona knows this fact too when you wrote "I snapped my eyes open when I heard her voice." then everything that follows implies Yoona's realisation but also her helplessness. She "chooses" to believe in the fantasy though she is well aware of the reality. You've definitely achieved your desired reaction from me with this fiction, you had me so surprised with every twist. At first, I thought they were living in an unhappy marriage, the way Yoona couldn't touch or say much. Then, when I realised that wasn't the case, I was extremely sad because it was much worse. But, you got me again when I thought they united, and then BAM! You threw me off my chair in the end!"

So what's your ending?