I am currently reading Victories of the Martyrs by St. Alphonsus Liguori and I came across something that would explain why one fails to carry one's cross during times of trials and tribulations.
"...The examples of the martyrs teach us also to have immediate recourse to the assistance of God, by earnest supplication, when we feel ourselves disconsolate or weak under affliction. Thus did the holy martyrs. As their torture increased, they multiplied their prayers and secured the victory...
...There have...been examples of Christians who, failing to invoke the assistance of the Almighty, have fallen off from the confession of the faith, and forfeited the glorious crown.
An example is found in the acts of the martyrs of Japan.
An aged man, having been condemned to a protracted martyrdom, endured the torture for a considerable time, but failing to invoke heavenly aid, denied his faith in a few moments before he expired--a startling warning to all, that perseverance in prayer, in times of temptation and distress, is that which alone can insure us the victory..."