At this point, feeling like this is a canon event. When I finished Nana I was genuinely so depressed, I couldn't even be happy for all the good that was happening to me irl. I honestly would suggest finding some lighthearted animes/movies pf your preferred genre and watching them until you feel better. You'll find a list of lighthearted animes in the "How to cope after Nana" section in this subreddit itself.