It is very common to experience these issues, and they are highly treatable. First, please visit a dentist, as active decay requires professional treatment, like fillings, before you attempt whitening, which can cause pain in damaged teeth. For daily care, use a fluoride toothpaste to strengthen enamel and prevent further decay. Brushing twice a day, flossing daily, and using a tongue scraper will significantly improve bad breath. You can also use an alcohol-free mouthwash containing cetylpyridinium chloride or a low-concentration chlorhexidine for freshness. Once your teeth are restored, professional whitening can safely address the yellowing.