A person should realise that no matter how grim the future seems to be, there is always light at the end of the tunnel.

grim 是形容词，意思是"严峻的、严酷的、令人担忧的"，常用来形容令人不愉快的、困难的情况或前景。这个词带有一种压抑、阴郁的感觉，暗示情况看起来非常糟糕或没有希望。