The pessimist sees only the tunnel; the optimist sees the light at the end of the tunnel; the realist sees the tunnel and the light - and the next tunnel.
— Sydney J. Harris

Struggling is hard because you never know what's at the end of the tunnel.
Don Rickles tunnels quote

Don't be too optimistic. The light at the end of the tunnel may be another train.
— Unknown

Unhappiness in a child accumulates because he sees no end to the dark tunnel. The thirteen weeks of a term might just as well be thirteen years.
— Graham Greene

If we see light at the end of the tunnel, it the light of the oncoming train.
— tunnels quotation by Robert Lowell