For many people, though, that feeling is more frequent, and they're plagued by a lack of self-belief and questioning.
"It's very common to see ourselves in black and white terms - so we are either successful or a failure," says psychiatrist and CEO of Thrive mental wellbeing platform, Dr Andres Fonseca. It may be common, but that doesn't make it right - or realistic, he says.
So, how can we push that niggling voice aside and have a bit more faith in ourselves?
Self-doubt is often deep-rooted from our experience as children. "If we grow up thinking we are good enough, this can lead to us to believing we are competent in our choices and confident in our abilities," says psychologist Dr Jane McNeill, from Clinical Partners. Similarly, "if we grow up to believe we are never going to be good enough, that is going to limit what we do, or make us think we don't deserve to do well, or have confidence in our achievements.
"We may find ourselves thinking or saying, 'I'm useless at this', or 'That was just a fluke'."
Wendy Smith, a personal development mentor, TEDX speaker and Paralympian, agrees: "We are programmed throughout our early years with those subtle comments that get stored at a deep level. These rerun throughout our adult lives, causing havoc and continual unsupportive thoughts, leading to that nagging little voice of doubt, which is always there when you don't need it."
Understand the power of self-doubt
Smith explains: "The brain does not distinguish between imagined responses and real responses, therefore when you say you can't, or you doubt yourself, it believes the imagery and emotions and makes it a reality. The human system is so powerful that it will support and bring to fruition anything you are currently running through your mind."
So, these beliefs can be really limiting, leading to missed opportunities and diverse life experiences, she says.
Think about what you're placing value on
Self-doubt is very much linked to self-esteem, and "how we rate and judge our worth in the world", says McNeill.
Basing core judgements of yourself on external factors, like your job, the way you look, your relationship status, the number of friends you have or the amount of money you make, doesn't equate to healthy self-worth.
Fonseca says: "If you think about it, none of them are likely to be essential to feeling good about ourselves. When any of these features become a foundation for your self-worth, it can lead to fragile self-esteem. As long as you see yourself through this lens, it will inevitably lead you to 'score keeping' - and you'll find that no score is ever high enough."
Build a fuller picture of yourself
Remember there are many, many ways to view yourself - we are "multidimensional beings", Fonseca says. You are not simply your job title, or your family role, you are an infinite number of things, which have infinite value.
"We need to know that we have many qualities, and achieve a sense of feeling that these qualities are valuable in different ways. The next job is to actually believe ourselves when we say these are valuable qualities that we should take into account when appreciating our own self-worth," he says.
McNeill agrees it's important to "recognise and enjoy your strengths, and make time for people in your life who validate your feelings".
"There will have been times in life when you succeeded and felt great with no doubt in your mind. Remembering these events can give you a head start on defeating doubt," Smith says, but go one step further and think ahead too. "Use mental rehearsal before you attempt anything.
"Imagine doing what it is and feeling how it feels to succeed. See yourself walking through the scenario time and time again with more confidence, hear yourself saying that you believe you can achieve and then really feel what that feels like."
The 'feeling' part is the most important part to focus on, she says. The more intensely you can feel that emotion, the more energy you'll have driving you forwards to achieve it.
Ask if it's what you really want
"We may find that we set challenges which are about making ourselves feel more acceptable to ourselves, and others which aren't actually about the person we are," says McNeill. So, each time we don't achieve that goal, we're caught in a never-ending cycle of knocks to our self-esteem. "This is because we're motivated by the fear of believing we're not good enough."
So, it's key to make sure what you're going for - a new role at work, a side project, a particular relationship, a solo trip or a fitness challenge - is actually fulfilling to you, to who you are deep down, rather than filling a need to feel accepted by others.
Fonseca agrees that listening to societal expectations can be dangerous. "When we measure ourselves with these standards, we might find ourselves lacking - and that's the problem. You need to embrace your own path.
"If you're able to shift that perspective and strengthen your sense of self, you will find yourself prioritising what really matters to you."
When things do go wrong, rephrase your response
Doubting yourself is very much entangled with the fear of failure, but what if failure wasn't such a bad thing? "Accept mistakes and accept we will inevitably make them," says Fonseca. "If you're not making mistakes, you're not really learning.
"The important thing is not to blame yourself, but to appraise what went wrong. That way, you can see yourself as a person that is forever getting better and improving, rather than someone constantly making mistakes and failing.
"This is how progress happens."