Suddenly believe --- you will get out of the tunnel? (3,3,5)
I believe the answer is:
see the light
If this is the answer, I'm afraid I cannot explain it.
Can you help me to learn more?
(Other definitions for see the light that I've seen before include "Come to understand" , "Become converted" , "Gain sudden understanding" , "Realise a fact or mistake - be religiously converted" , "Realise one's mistake" .)