Like you, I don't know how or when your marriage began to fall apart and even if I did, I would still urge you not to spend all your time focused on the past.
Don't misunderstand me; it's important to learn from our past and discover what we did wrong so we won't do it again. But your real focus needs to be on the future, and what God wants to do in your life in the months and years ahead. After all, you can't change the past; you can only change the future with God's help. God's promise is for you: "'I know the plans I have for you,' declares the Lord, 'plans to prosper you and not to harm you, plans to give you hope and a future'" (Jeremiah 29:11).
What should you do? First, realize that God knows all about your situation, and He loves you and wants to help you. More than that, He wants to cleanse the past and make you part of His family forever. Ask Christ to come into your life today.
Then ask God to help you leave the past behind, and learn to walk with Him every day. The Bible says, "Cast your cares on the Lord and he will sustain you" (Psalm 55:22). In addition, ask Him to help you get rid of your feelings of failure and guilt. Your husband is responsible for what he has done to break up your marriage, and you should not bear the blame for his failures.