The Bible identifies the unpardonable sin as blasphemy against the Holy Spirit. While interpretations vary, many theologians believe this refers to a deliberate and persistent rejection of the Holy Spirit's testimony about Jesus, accompanied by attributing the works of God to Satan.
Simply rejecting Jesus is not automatically the unpardonable sin. Many people reject Jesus at some point in their lives, but later come to faith. The unpardonable sin involves a hardened heart and a conscious, malicious rejection of God's grace.
Some denominations teach that the unpardonable sin is only possible during Jesus' earthly ministry when people witnessed His miracles firsthand and attributed them to demonic powers. Others believe it is possible today, though difficult to commit unknowingly.
If you are concerned about committing the unpardonable sin, it is likely that you have not committed it. The very fact that you are worried suggests an openness to God, which is the opposite of the hardened heart characteristic of this sin.