Yes, God understands your situation, and He knows what you can and cannot do at this stage of your life. Thank God for what you were able to do in the past and thank Him also that you can still give something to support His work. We should never feel guilty over things we can't do.
Remember too that God is able to use even the smallest gift, when it is given in the right spirit. On one occasion, the crowds following Jesus had nothing to eat, and they were miles from the nearest town. But a boy gave Jesus his small lunch and Jesus miraculously multiplied it into enough to feed 5,000 people (see Matthew 14:13-21). Pray that God may do the same with your gifts.
Or do you remember those Jesus observed entering the Temple? Some of them were rich and threw large offerings into the treasury box (perhaps hoping to impress others with their generosity). But then a poor widow put in two small coins, worth less than a penny. Jesus commended her to His disciples: "I tell you the truth, this poor widow has put more into the treasury than all the others. They all gave out of their wealth; but she, out of her poverty, put in everything all she had to live on" (Mark 12:43-44).
Ask God to help you be wise in your giving, and to help you give all you can. But most of all, ask Him to help you find new ways to serve Christ in your retirement.