I love this concept! The logline is okay, but I feel like there is something missing.

I would add a twist as a midpoint reversal to change the direction of the story. I think there is too little conflict in the logline right now or maybe it is not visible. What are they going to do as a couple after they fall in love? What is standing in their way?

I understand your last sentence “only to find out she never existed” as the beginning of your story, aka plot point I.
But as dpg already stated, what is the objective goal of your protagonist after he finds out she never existed. What does he have to do?
Right now,? with this version, there is no road map and clear intention in what kind of direction your second act is going for.
Does he want to prove that she exists (existed)?
Often, the best solution to this problem is by defining the theme of your story.
Example 1: My story is about delucions that can be so strong that you literally think they are real.
Example 2: My story is about second chances and how everybody should be able to make up for his first failures.

So, what is your story (logline) really, deeply about?
If you know that question (or answer), you have your logline- and, thus, your story.

Hope that helped.