In John 19:17, Jesus is described as carrying his own cross, but in the other gospels we are told that Simon of Cyrene carried the cross for Jesus (Matthew 27:32; Mark 15:21; Luke 23:26).
It was customary in Roman crucifixions for the prisoner to carry his own cross. Most likely, he had to carry only the crossbeam as the upright beam usually remained in the ground at the place where the executions were held.
Putting the four gospel records together, it seems likely that Jesus started out carrying the cross, but because of the suffering he had endured he was too weak to continue, and so the soldiers forced Simon to take over.