If the adult child cannot care for him or herself, then you still run the show. You have a responsibility to that child if he/she is still dependent on you, but that means you have to consider all his or her needs, including the financial ones.
If your money situation requires you to work, then it WILL be hard. But there are ways to make it easier--have a sitter, or work from home. See if local bus companies will allow you to bring your child to work with you--then you can get a job there.
See if your child can be a part of your work. On a bus, he or she can help with the passengers. If you babysit, he/she can help with the kids. Could he or she help with a cleaning job? If you become an Avon, Tastefully Simple, Tupperware, Mary Kay, or Discovery Toys Rep, he or she could help sort, hand things out, demonstrate products, etc.