Originally I thought it would be melancholy, but I guess that is more of like a pensive sadness (like sitting inside watching a rainy day listen to nostalgic music). It's not quite bittersweet because that is more of the feeling when one walks out of a friend's house after a moving away party.
Further, depressed and the like don't fit what I am looking for as depressed is a longer term and more serious thing.
Example: When I looked out at my family and friends at my birthday from my balcony I knew I should feel elated but I just felt _________ (melancholy mixed with bittersweet; a little numb as well) even as my lips gently curled into the faintest smile.
This would could also be used to describe the feeling of knowing rough times are ahead but still content when a wiser person/character is listening to a young, enthusiastic child who talks positively, excitedly, albeit unrealistically optimistic about the world.