You need to practice. Competency Based Interviews are easy to prepare for. They are just you telling stories.
They generally follow a standard format - tell me about a time when: you worked on a team, you had a difficult client, you had too much work to manage and needed to prioritise, you had to take a risk, you had to tell someone something they didn't want to hear.
Then the follow up questions are generally something like - what did you learn, what would you do differently etc
PRACTISE. Practise in the car, in the shower for 10 mins before you go to sleep. Look back through you CV and write your stories down. Learn them. Practise with a friend, aunt, brother or mentor, whoever really. Interview them back so you can hear their answers. Practise all of the time till you are sick of your own stories.
On interview day, take your CV in with you and a list of questions for the interviewer. Always ask for a glass of water, if you freeze you can buy yourself time having a sip.
If they ask you a question you don't have a prepared answer for then don't go silent. Say "that's an interesting question, I need a minute to think that through" Then flick through your CV and see if it prompts a memory. Keep talking - say something like, I don't have an example for that, but something similar happened, can I tell you about that. If the interviewer moves on, then move on too, but ask to go back if you think of a good answer.
You have skills that organisation are looking for, they invited you because your CV demonstrates you have those skills. They WANT YOU before you step in the door so just show them your amazing self. 