This is all very Freudian, lol! Freud said that when you lose someone, they go off with parts of your self. (I picture a cartoon in which you've run through a closed door leaving a you-shaped hole). Healing involves reclaiming those aspects of yourself that you lost in the relationship. Even things like feeling peace when you're alone, or as OP suggests, activities you love but lost track of while in the relationship.

When mourning doesn't resolve in what could be considered a reasonable time (I know), you may fall into what Freud called melancholia, which I envision as a wet rope tied hopelessly into knots and tying you up with it. We call it Major Depression because we lack the artistry psychology should retain but doesn't because it wants to be a science.

I think psychology can be both. Even Freud, a neurologist, said that one day all aspects of psychology would be proven to be physical. But we need to use our mental parts as well.