A "bittersweet ending" refers to a conclusion or outcome that contains both happiness and sadness at the same time. It is a mix of positive and negative emotions, like tasting something bitter and sweet together. Full definition
It all builds to a massive bittersweet end which will leave your mouth agape as the final credits roll. (thesixthaxis.com)
The kids have started their countdown until summer vacation starts, and to be honest so have I. It's always bittersweet ending another school year and watching your kids grow up right before your eyes — but I truly look forward to not having to set an alarm clock or pack lunches or being on any kind of schedule for a couple of months. (aggieskitchen.com)
It's like she's there only to set up a not very effective sentimental backdrop which culminates with a rather unwarranted bittersweet ending. (montrealfilmjournal.com)