Okay, cool. Does the relationship end badly? That can also contribute to a “downer” feel.
I think the best thing to do would be to look for analyzed Success/Good stories that have this sort of high Cost, slight downer, ending, and see if they feel anything like your story. You could solicit help from others on this board in finding those (how high the Cost is isn’t something you could tell from just the storyform, so you would need to be familiar with the story).
Braveheart is the quintessential example of high Cost. I’m thinking Blade Runner: 2049 might also be a good one just because there was some discussion in this thread around whether the Judgment was Good or Bad (I haven’t seen that film yet though! 
)
EDIT: @crayzbrian also mentioned Black Swan above.
Also, are you sure your Outcome is Success (Goal achieved)?