Never mind whether the negative idea you hold is true. Ask the other question: what does keeping it cost? What if you believed your luck was good, or that you are beautiful, or that you are loved, or that so-and-so's opinion doesn't matter in the slightest and was never worth taking seriously - or that you are a failure at your studies?
Write about what the belief, or the negative thought, stops you from doing. Write about the steps postponed for years, the parties and the gatherings you excused yourself from, the chances you lost because you believed you were incapable, a failure, unfit, unsuitable, not presentable, not acceptable in your looks. And write about what that belief gave you. What did the excess shyness give you - the times you wanted to speak and didn't, because you believed your accent was bad, or that your standing didn't entitle you to speak?
Each day, after that short trial, decide in your writing whether you are keeping the belief. Then write about what you did that day that doesn't fit the negative belief about yourself, and how things went well and the belief turned out to be wrong when something actually happened.