Maybe it's these three, then: 'ave atque vale'...

By themselves, they just mean 'hail and farewell'. But at the end of Catullus' poem, they are the climax of one of the saddest of all poems. As a young man, I heard these words for the first time before I really understood them, and then I experienced a few instances of grief: a distant friend, a grandparent. These were people whose lives I shared in some small way, briefly or just for a few moments every now and again, but not ones who I spent a lot of time with. Looking back, these were cases where I could say the words - and know what they meant - without really feeling what they meant to Catullus himself when he wrote them. 
Making sense of grief is, by the evidence of the poetry, the songs, the paintings, the books, just about the hardest thing that people have to do. That, and when love does not work for whatever reason. These seem to be the foundations on which art is built. I can't paint, my poetry (usually in the form of song lyrics) is pedestrian at best, my singing is bad, So, I write. On here, These Matters, the flash fictions, all - give or take - about making sense of grief, over people lost and love that cannot work. 
Christina Rossetti's poem - the one that gets read at all those funerals - counsels that it is better to forget and smile than to remember and be sad. If only it were that simple, although I do not offer that as a complaint about her advice. I recognise the sentiment: I should not feel guilty if, after time, when distractions are there, I do not feel that grief at every moment. Life, as my fundamentally unemotional dad said to me at his own mother's funeral, goes on. But life is irretrievably altered by those people we lost, whether that is to death, or to the long desperate future of life without them whilst they are still out there somewhere, which is the central theme of book IV of These Matters. 

I can't really express the feeling. So here it is in the words of someone better at expressing it, and whose sadness ultimately took his own life:
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I, I never wanted to write these words down 4 you
With the pages of phrases of things we'll never do
Hey, so I blow out the candle, & I put you to bed
Since you can't say to me now,
How the dogs broke your bone,
There's just one thing left to be said...