God knows your limitations, and He knows you can't do everything you once did–nor does He expect you to. But as long as He keeps you on this earth, you aren't useless! He still has a purpose for you, and I hope you will seek it.
What is God's purpose for you at this stage of your life? I don't know the full answer, of course (which is one reason I encourage you to pray about it). But has it ever occurred to you that you are serving Christ just by making the effort to go to church? Young people, for example, are observing you, and you are teaching them the importance of hearing God's Word and being with God's people. Someday, they too will be old, and they'll look back and remember your example.
But you can serve Christ in another way–and that's through prayer. One of the most important ways we can help others is by praying for them. As he grew, older Samuel couldn't serve God's people the way he once had, but he told them, "As for me, far be it from me that I should sin against the Lord by failing to pray for you" (1 Samuel 12:23). My own ministry would never have been possible without the prayers of countless older people.
Thank God for each day He gives you, and ask Him to use you to point others to Christ. And thank Him most of all for the hope we have of eternal life through Christ.