It's possible to go on, no matter how impossible it seems, and that in time, the grief . . . lessens. It may not go away completely, but after a while it's not so overwhelming.

This quote means that even if it feels like it’s impossible to keep going, it’s still possible. Eventually, the grief that you are feeling will become less intense and although it might not go away completely, it won’t feel so strong. Over time, the grief becomes more manageable.