You are experiencing what millions of others have experienced (including my wife and me): the so-called "empty nest." And yes, it's a time of great adjustment even of stress for some couples. But with God's help it can become one of the most fulfilling times of your life.
We sometimes forget that we don't have children just for our own enjoyment or entertainment. Instead, our goal as parents should be to prepare our children for adulthood physically, mentally, emotionally and spiritually. In other words, our goal should be to prepare them for the time when they will no longer be with us, but will leave to form their own careers and families. This is the main reason God has entrusted our children to us. The "empty nest" is part of God's pattern for our lives.
How should you respond? First, ask God what His will is for you during these years. Let the psalmist's prayer become yours: "Show me your ways, O Lord, teach me your paths" (Psalm 25:4). Much of your life has been spent helping your children; are there others in your church or community you can be helping now?
Take time also to strength your relationships with God, with others and with your husband. When we're busy, we tend to take others (including God) for granted, and later we find those relationships have grown weak. Don't let this happen to you.