I can't stop copying peoples' personalities when I'm in social situations... I always feel so insecure. Help?

One can not always please everyone. You need to believe strong enough in yourself and what you are passionate about, that you are willing to take a stand and be yourself, and maybe not please as many people. One can increase one's confidence in oneself by developing your interests, talent and skills to more of a degree so that you know more of what you are capable of. Mind-body exercise, strength training exercise and aerobic exercise will also help build your confidence more. One does not need to exercise a lot, just to do it regularly. Meditation on any emotions that give you trouble will also help. Just find a comfortable position, breath deeply, tensing your stomach muscles a little when breathing in, relaxing your stomach muscles when breathing out while focusing on any emotions or images that you are uncomfortable with. You can close your eyes to help you focus. Meditation is like an exercise, and can be difficult to do at times, but if done regularly will bring results in time. One can also get more connected to the serious problem and issues affecting this world and affecting people. Feeling connected to the intensely emotional issues of the time will increase your passion for things. With increased passion and confidence, you will be a lot less concerned with being someone else, and more into representing your perspective and personality.

Practice exerting yourself. Make sure that what you do or say reflects your personal thoughts/opionions. I am guessing you are young-so it is a developmentally normal thing to do. Just remember to be true to yourself. Try going against the grain some time- like a social experiment. Have you ever shouted out something in a public place-like if you were out with your friends somewhere and you said something really loud-Like "No, I won't make out with you!" People will stare at you, you may get a funny look or two, but are these strangers opinions what matters? No-you are what matters-so just be yourself.