one-upper downer

(n.) Someone who tries constantly to out-do your bad personal experiences with their own bad stories, which in their eyes, is always worse.

Me: Oh man, I just had the worst day at work! I got fired...

Tim: Oh, sorry about that. Mine wasn't too much better. I forgot to save a really long document before shutting down my computer. Such a bad day!
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Tim's a clear one-upper downer