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I am sorry you are having difficulty. It sounds as if you have symptoms of Social Anxiety Disorder. This is a type of anxiety disorder that creates excessive fear of social situations. It may make you feel self conscious about being criticized or judged by others, and then results in making mistakes socially, feeling embarrassed, which only reinforces your internal beliefs about yourself as inadequate. This continues to create a cycle of fear in social situations, and you may avoid them completely or feel tremendous anxiety in social situations. Anticipating social events can also create undo stress. Although these may be distorted beliefs, they feel real to you. So it is the beliefs about one's self that must be changed. These beliefs can influence you not only socially, but occupationally, at school, etc. Anywhere there may be people.
You are doing a lot of the right things, practice helps. Consistency is the key to helping with this. Sticking with one thing for about 21 days helps the brain develop a habit. Habits become second nature and are easy to develop, just ensure it is a positive habit. For example, setting a goal to talk to one new person per day, even if it is stupid or cloche can help you develop more confidence in starting a conversation. Sometimes the new person may not be as receptive as you might want them to be, but do it anyway, because the goal is for you to feel comfortable, regardless of their response.
Ideally, to assist you with this, it would be recommended you start a medication for anxiety type of symptoms such as a beta blocker or benzodiazepine. Both of these medications can lower heart rate and make you feel calmer in social situations. This will assist you in developing more skills in social outings without feeling the physical symptoms that may accompany.
Getting some assistance through a professional counselor will help also, as he/she can give you specific exercises to practice and you have a support person to bounce things off.
As for the family up bringing, it could have something to do with the dynamics of the family and parent interaction. If your sister is more sociable, it may be that girls learn this trait a little earlier, as they are expected to carry on superficial chit chat with others at an early age, thus becoming more comfortable with it.
The whys don't necessarily matter right now. I think if you work to correct the symptoms you are having, you can then identify more of the cause at a latter date.
You can contact your regular MD to assist you in getting started on medications, and you can keep this confidential from your Mom and sister, if you want.
Please let me know if you have other questions. Thanks, Gina