You're right, of course; we can't really say, "I forgive you" to someone who is no longer alive. This is one reason why I urge people not to carry their burden of anger and bitterness any longer, but - with God's help - to get rid of it by forgiving those who've hurt them. I realize, however, that this wasn't possible in your case.
No, you can't forgive your late husband for what he did, but you can turn your pain and your hurt over to God, and let Him lift them from your shoulders. God knows what you've been going through - and He loves you and wants to take this burden from your heart. The Bible says, "Cast your burden on the Lord, and He shall sustain you; He shall never permit the righteous to be moved" (Psalm 55:22, NKJV).
How can you do this? First, if you've never done so, turn to Jesus Christ and ask Him to come into your life. He loves you, and when we come to Him in repentance and faith, He makes us part of His family forever. Then ask Him to take this burden from you, and to take away the hurt and anger you feel.
In addition, ask God to guide your future. It's easy to get stuck in the past - but don't let that happen to you. Instead, you can face the future with hope, because Christ is now with you.