I saw that you said that you don't now how to get to the root of this negative thoughts. I kinda had the same problem. Lemme tell you how I overcame it.

Back then I would have the fear of feeling negativity. I'd be living in the end, and a negative thought would come up and I would almost strain myself to suppress it out of fear. This made me feel weak, and therefore made me weak

What I do now whenever it happens is downplay it. More and more I began to see these negative thoughts as an annoying little brother or sister that was hopelessly trying to overpower me. A powerless, helpless and vain thought.

Whenever you have negative thoughts, just breathe deeply. Don't let it scare you, and tell yourself nonchalantly "It is impossible for me to fail, nothing will ever change that".

Neville said to relax into the state that you desire, but you should also relax into the knowledge that you are absolutely powerful. Thoughts like that will seem more and more trivial, and you will have less of them if you succeed in doing this.

Google the Pharoah meditation. It helps in developing a feeling of absoluteness

A good start would be to really delve into why you do not feel confident. Do not force the feeling away. Look into why you are feeling this why and understand that the circumstances that made you feel that way will not be present in your new life. Imagine it to be with another version of you that you have left behind. A version of yourself that has died to you. You are new again

I hope this somewhat helps