It's important to understand that confidence can come and go. If you feel that your confidence has taken a knock - that's fine. There are things that you can do to build it up again. If you feel overwhelmed or uncertain, that's fine too. There is always a way forward.
You may be confident generally, but find in certain situations that you 'wobble'. Or it may be that you have always lacked confidence in yourself and your own abilities. You may not even like yourself very much, although this may be due to low self esteem.
If your life has changed in any way, you might find that you feel less confident than you used to. For example, you may have been ill or in hospital for a while, or maybe out of the workplace for a period of time. You may have lost your job, or not passed exams that you needed. Maybe you look different, for example you could have lost or put on weight. Perhaps you are going through the menopause and are surprised by how you feel as your hormones fluctuate. Or perhaps you have experienced a loss of some kind.