Joylorn

Joylorn (adjective): feeling both happy and sad at the same time, usually as a result of experiencing something pleasurable that is also bittersweet or tinged with melancholy.
The term is a combination of "joy" and "lorn," which means forlorn or lonely.

She felt joylorn as she watched her children grow up and leave the nest, happy for their success but sad to see them go.