I have phobias that are intense. Would it help to tell my parents, or make it worse?

Yes, do tell! Depending on type of phobias, there are good medications for treatment. Sometimes, infections and/or immune system activation can contribute to emotional imbalances as antibodies affect the basal ganglia of the brain to present with emotional instabilities, obsessive-compulsive behaviors, and even movement disorders.
Helps. When someone has an intense problem, it is very helpful to share that with those who love you and want to help you. Joys shared are doubled and burdens shared are halved. There are some excellent counselors that can help you with your phobias. Homeopathic care can also help. Look for a homeopathic practitioner near you as well as a counselor.