I once heard someone say, "When satan reminds you of your past, remind him of his future."
We all have a past. We have all said or done things that we wish we're not said or done. But maybe some of those things moved us along to where we are now.
Many people in the Bible had "pasts" or behaved in ways that were less than ideal, yet God used them and became very well known.
What can wash away my sin? Nothing but the blood of Jesus.
And though our sins were as scarlet, we can be white as snow.
Our sins are then as far away as east is from the west. Leave them there. Don't listen to the deceiver.
Even now, as Christians, we sometimes fail. Maybe even in big ways. And then satan tries to convince us that we are not worthy to have the love and forgiveness of God. But remember that while we were yet sinners, Christ died for us. And God is not willing for any to perish.
Who are listening to? God saying, "I love you, come to Me" or the deceiver telling you, "it is no use?" I choose to listen to the voice of God.