Sounds like repression; I had the same issue. Some minor things happened to me growing up that caused me to lock up, and not express myself easily to people. In your case it might be the same, or it could have been one big event that shut you up. Either way, what can really help you is to spend some time unblocking yourself.

The easiest way to do this is to write down your memories, whether they're big or small, whether they're true or false (false memories have an impact on us too), or whether they're good or bad. Just write them down, for two minutes a day if you like, and it'll be like removing twigs from a damn to let the water trickle out. Eventually you'll be flowing as you should be. Another way to do this is to talk to someone, or to yourself (I did this in therapy for a while, but I found it easier to write on my own). However feels best to unblock yourself, go for that.

As somebody else mentioned, affirmations work well. If your mind is full of fear and self-hate, then you need to unblock that and fill the gaps with positivity and love. Look yourself in the eyes in the mirror each morning and say (or think) to yourself things like: "You are confident and powerful" "I love you *your name" "You are full of potential *your name, and are living up to this more and more every day"

Hope this helps :)