In the end, only three things matter:
how much you loved, how gently you lived,
and how gracefully you let go of things not meant for you.

--attributed to the Buddha,
and paraphrased by many

Never give up on something you can't go a single day without thinking about.
--Anonymous

(I am suspicious of the word "never". It goes right in with "always" and "should". And I notice that after a while, the thought occurrences of a person who is gone do decrease. Maybe never completely gone, but at least not so torturous. I do however think that this quote, as a mantra, is destructive by encouraging obsessive thought, and will impede the process of letting go, and hence I apologize for posting it. I just had to.)