On the outside: I’m a b*tch. I use it as my shield because I have been in bad situations. I have a perverted sense of humor (part of that is really me, but…). People think I am very angry/morbid, though my friends don’t think so.
Inside: I’m going into nursing. I want to join the Peace Corps then become a foster parent. I want to volunteer at soup kitchens and such, but I am terribly shy (something you can’t tell from the outside).
How do i get the two to match up. Please don’t tell me “just forget about what everyone says and do what you want to do.” It’s not that easy.
Translate Post: 
Don’t just “want” to volunteer at soup kitchens. Start doing stuff like that now, and by helping other people you’ll feel better about yourself. This, in turn, will help you greatly in shedding your “bitch” label.
stop beinging a b*tch ( jk i dont know if you are a b*tch or not( but u if you are stop being one))
If you can’t stop being a b*tch, (everyone needs a shield!), then try to smile or wink after you’re mean, just to say “Just kidding!)
It is common with everybody. There is nothing special. Let us discuss this issue coolly tomorrow. Postpone for the present.
People use certain features as shields like you said cuz they have been in bad situations. Just start volunteering at the soup kitchens and put yourself in those situations that make your shy side uncomfortable. There isnt really just one answer to your question. Just try to tone down your b*tchy side and up your shy, nursing, soup kitchen side. Put your plan in action too! If you are the right age you can start volunteering at the Kitchens! Very respectable wants for your future. Best of Luck with everything!
It seems like a self-esteem issue. I like what the first guy said, once you get out there and really help out, you will definitely see a change in the self-worth you give yourself. Another thing is people usually surround themselves with individuals that are like themselves. Look around you and see if there are people like you who are sarcastic and angry, and if that is the case beware because even jokes can, overtime, make someone believe certain things. Try to find some mentors, preferably with common interests, that you look up to. For example, take up a hobby, and do it with someone that has great self confidence. Or just surround yourself with more positive people in general. I’m not suggesting you change your friends, just add to the bunch to spice up the mix. Try also talking about how you feel with the people that are really close to you. Some of the best insiders views i have gotten from people that know me well. They may have answers to why you have put up this wall, how you show yourself to the world, and how you can break through these habits.
Try to go slow, find ways to get to know yourself better, and find a way to trust other people again. Once you gain back that trust, and see that there are people out there who are different from the ones that put you in those difficult situations, you will realize that you can open up to the world again. Once your self-esteem grows, and once you can bring down your wall, you will be stronger against those people who are destructive as well.