When we know we've hurt someone, we ought to seek to make it right–and the only way to do that is to admit we were wrong and ask for their forgiveness. This isn't necessarily easy–but it's the right thing to do.
After all, this is what you did when you came to Jesus and put your trust in Him. You knew you had sinned against Him, and you knew you needed His forgiveness. But you didn't stop there; you also turned to Him and asked Him to forgive you. And He did forgive you–not because you deserved it, but simply because He loves you. He loves you so much that He gave His life for you.
In a similar way, Christ wants you to try to make things right with those you have hurt. When the prodigal son returned home after abusing his father's trust, he knew his first words needed to be, "Father, I have sinned against heaven and against you" (Luke 15:21). His father immediately forgave him (which is a vivid picture of God's forgiveness of us). But this also reminds us of our need to seek forgiveness from those we hurt.
Ask God to help you write a letter to your former spouse asking for his forgiveness. Make it clear that you have given your life to Christ, and express the hope that someday he too will turn to Christ. As a footnote, let your present husband know what you're doing so there won't be any misunderstanding.