How to shake years of social withdrawal and be more outgoing?

Therapy can help. Reversing social withdrawal depends on a number of factors. All of those factors could be discussed with your doctor/therapist. Some cases of social withdrawal are caused by anxiety and shyness, others are caused by a reaction to traumatic events that makes it more difficult to be in certain social situations. Regardless of the cause, people can learn to gradually increase their socializing.
Seek help. Ideally see a therapist, counselor, or psychologist to help uncover underlying reasons for your social withdrawal and help you formulate a treatment plan. If you are financially unable to do so, church based activities may be your next best bet. Church based counseling is generally free of charge, and church based social activities are generally very welcoming even to shy and withdrawn newcomers.