I find a surprising number of people do what you did: They plan to make drastic changes in their lives after they retire, but they never stop to think realistically about the difficulties they may face. Perhaps your letter will encourage them to reexamine their plans.
What should you do now? The most important thing I can say is to urge you to seek God's will for your future. Ask Him to give you wisdom and guide you as you face the future. Be open to whatever He has in store for you, no matter what it is. Let the psalmist's prayer be yours: "Show me your ways, O Lord, teach me your paths" (Psalm 25:4). God loves you, and He knows what is best for you. Trust Him to show it to you.
In the meantime, make it your goal to make new friends right where you are. One of the best places to do this is through your church–and if you haven't yet found a church home, ask God to lead you to one where you can grow both spiritually and socially. Other retirees in your community probably feel the same way you do, and together you can make a new life.
In addition, ask God to help you make these the best years of your life, by giving you new opportunities to serve Christ. When Christ is first in our lives, He gives us a purpose for living every day. Ask Him to show it to you, no matter where you are.