The ACL does not heal. Either it is reconstructed surgically, or the injury is compensated for with a combination of bracing, activity modification, and exercises to build up the strength in the surrounding musculature. Exercise should focus on building up the strength in the hamstrings. These muscles pull the tibia backand help to compenste for the laxity created by an injury to the ACL.
Although ACL injuries can be treated non-surgically, if you are young, active, and significantly unstable, it is likely that conservative management will not provide a satisfactory outcome. You will likely experience further and worsening instability, and with each "giving way" episode, you are at risk for injuring other structures in the knee, including the menisci, the articular cartilage, and other ligaments.