Supply issues can be one of the most stressful problems when breastfeeding. It is easy to feel that you are letting your child down, which can lead to guilt setting in.
Everyone's body will respond differently to the demands of breastfeeding. Some may produce too much, or too little milk. Training your body towards whatever your goal is can be done in most cases.
If not, you can supplement with formula. When it is time to wean then off then this can be an important part of the process too.
The below posts will help answer your questions, and more.