I don't know the reasons why God keeps you around -- but God does, and the most important thing you can do is ask Him to show them to you.
What might some of His reasons be? One could be to demonstrate to others what it means to trust Christ and live for Him every day, in spite of your problems. What will others (including your grandchildren) remember about you? Will they remember you as someone who always complained and was hard to be around? Or will they remember you as someone whose life demonstrated Christ's peace and joy and love? The Bible says, "The joy of the Lord is your strength" (Nehemiah 8:10).
Do you remember Job in the Old Testament? Job had everything a person could ever want -- wealth, a large family, good health. But all that was taken away from him, and at times Job felt exactly like you do (if not worse) -- useless, burdened by pain, alone, filled with doubt and despair. He even cried out, "Why did I not perish at birth, and die as I came from the womb?" (Job 3:11).
But eventually Job learned that God had a purpose in his suffering, and through it all he came to see God's greatness and love. In time, his burdens were lifted, and his life was restored. In a far greater way, we have hope, because of Jesus Christ and what He has done for us. Make Him the center of your life -- beginning today.