The symbol of the cross, and variants of it, were used well before Christianity, at least according to the Catholic Encyclopedia:

Various objects, dating from periods long anterior to the Christian era, have been found, marked with crosses of different designs, in almost every part of the old world. India, Syria, Persia and Egypt have all yielded numberless examples… The use of the cross as a religious symbol in pre-Christian times and among non-Christian peoples may probably be regarded as almost universal, and in very many cases it was connected with some form of nature worship

It actually wasn't used particularly often in very early Christian worship or iconography. It was indeed seen as a symbol of suffering to the Early Christians, pre-200CE. Instead, similar symbols known as the Staurogram, Chi Rho, IH Monogram, and IX Monogram were used as symbols meant to represent the cross. Chi Rho was the symbol used by Constantine I.

By the Second Century, the cross as you know it today began to gain popularity (obviously before Constantine I), but variants were still widely used.

Also used to represent Jesus were the Good Shepherd and the dove, seen in early religious art and wall drawings.