He got involved with anti-Christian extremists who beat up Christians and sometimes killed them. He enjoyed this because he felt he was superior to Christians.
Then God spoke to him and he accepted Christ.
But how could he get over his past?
He writes about this problem below:
"I am the least of the apostles and do not even deserve to be called an apostle, because I persecuted the church of God... But one thing I do: Forgetting what is behind and straining toward what is ahead, I press on toward the goal to win the prize for which God has called me heavenward in Christ Jesus" (1 Corinthians 15:9; Philippians 3:13-14).
Just like Paul, there's nothing in your past that can keep you from loving and serving the Lord fully if you face it honestly and "press on."
For a very thorough 1-page explanation of Paul's pre-Christian life, I recommend this resource: Paul's Conversion. It contains both Scripture resources and commentary.