When doubts creep in before the viva they can be difficult to remove. By the time you recognise them for what they are it may feel like you simply have to feel them and feel bad. There is hope though: you can’t simply press a button and change how you feel, but you could do something or a series of things that would help the situation.
If you doubt that your viva will go well, then find out more about the viva. Look at the regulations, ask questions about other’s experiences and talk to your supervisor about what to expect.
If you doubt that you’re good enough or that your thesis is good enough then reflect on how far you’ve come. Write down a list of your successes. Write down a list of things that you know have improved in your ability as a researcher. Talk to your supervisor or others to get their reflections.
If you doubt you can speak in the viva, or if you doubt that you will be able to answer questions, then practise. Rehearse with a mock viva. Present your work and take questions. Do things, as uncomfortable as they might be at the time, to build up your confidence and comfort for meeting your examiners.
It’s very human to doubt. It’s not wrong to worry. But you’re not alone and you’re not without hope. If you feel doubts then act to remove them.