bittersweet - adjective \bit-ter-sweet\ : something that is bittersweet; pleasure alloyed with pain, combining happiness and sadness.
example: Celebrating a year of amazing accomplishments by my students as they get ready to move on to second grade, but also having to say good-bye to the best first grade year ever at the same time.
We had such a smashing time at our end-of-year party celebration. Food, fun, friends, movies, music, and stuffed animal buddies were had by all! And while seeing the children laugh and have a total blast made my heart smile BIG, I have to admit that I was also sad that this school year was over... it was most definitely a bittersweet moment.