One of the basic wishes most parents share is that their children get along, play, and interact well with others. Most often, these social abilities develop intuitively. But for children who instead struggle with peers, the earlier interventions start, the earlier they catch up. Since both ADHD and autism impact relationships, sorting out the 'why' behind social difficulties is often a first step towards figuring out how to best support this vital area of development.
Realize that there's a difference between bitterness and anger. As Maya Angelou so eloquently put it, "Bitterness is like cancer. It eats upon the host. It doesn't do anything to the object of its displeasure." Contrast this with anger -- it's okay to be angry, as long as you channel that energy productively.