se mettre la rate au court-bouillon

to work oneself up over nothing

The spleen is not an organ anyone has strong feelings about, which is what makes poaching it in a fish stock such a good picture of fretting over nothing. French does this a lot, taking it out on the blood in 'se faire du mauvais sang' and on the bile in 'se faire de la bile'.

Arrête de te mettre la rate au court-bouillon pour un simple retard de train.

Stop worrying yourself sick over a delayed train.

'La rate au court-bouillon' was the title of a San-Antonio novel in 1965, and the expression itself turns up in 1970 in 'La méthode à Mimile', the slang primer by Alphonse Boudard and Luc Etienne. Whether Frédéric Dard coined it or picked up something already in circulation is not settled.