I would use:

>Sei keine Memme

You can find more words in [leo][1].


Perhaps you could also use:

>Reiß dich zusammen (Pull yourself together!)


  [1]: http://dict.leo.org/ende?lp=ende&lang=de&searchLoc=0&cmpType=relaxed&sectHdr=on&spellToler=&search=wuss