Loss can make gratitude and sadness occupy the same sentence.

Grief and gratitude are not opposites, though they are usually filed that way. Being glad someone existed and being wrecked that they are gone are the same recognition seen from two angles. Trying to feel only the grateful half is a kind of self-editing, and it rarely holds. The two arrive together because they are describing the same fact.

If someone tells you to focus on the good times, you can do that without dropping the sorrow. Try saying both parts in one sentence and notice how much more honest it feels. Whole feelings are easier to carry than edited ones.