God understands what you're going through - and He is with you and your wife today just as much as He has ever been. You can trust God's promise: "The Lord himself goes before you and will be with you; he will never leave you nor forsake you. Do not be afraid; do not be discouraged" (Deuteronomy 31:8).
How does God want you to deal with this difficult situation? First, with love and compassion. When you were married, you took a solemn vow to be faithful to each other and help each other "in sickness and in health ... as long as we both shall live." Those words are often ignored today - but they teach us one of the reasons why God gave marriage to us. Don't get impatient with your wife or berate her for not remembering something; she can't help it - any more than you could in her situation.
Then ask God to give you wisdom and hope. Speak honestly with her doctor about what you can expect in the months and years ahead, and plan accordingly. It's hard to watch someone we love decline, either physically or mentally. But never forget: The Christian knows this life is not all; ahead of us is heaven.
Finally, ask God for strength - physical strength to take care of the demands you face, but also emotional and spiritual strength. Remember the Bible's promise: "The Lord is with me; I will not be afraid" (Psalm 118:6).