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painful; bitter; heart-breaking; difficult (emotionally)
tough; difficult; hard (usu. of situations)
cruel; harsh; cold
「辛い」 has other readings with different meanings.
毎日がつらくて仕方ない。
Every day is unbearably tough.
毎日働くのは辛いですよ。
Working daily is tough.
つらい時は無理をしなくていいよ。
When things are tough, you don't have to push yourself.