The simple explanation of what imposter syndrome is: feeling like you are a fraud and not deserving of all that you have accomplished. People with imposter syndrome are typically high achievers who accomplish a lot, yet they don't believe they actually deserve it. Despite things like education, hard work, experience, and everything else that would help them accomplish things, they still have self doubt about the accomplishments.
You may have imposter syndrome if you are unable to realistically assess your abilities and skills related to your performance and success. Common personality traits linked to people with imposter syndrome are perfectionism and low self esteem.
Some things that may help:
Understanding that you are experiencing this is the first step in helping yourself get over imposter syndrome.
Focusing on what is really going on and being able to distinguish the truth from how you are feeling.
Acknowledging everything you do to achieve your accomplishments will remind you that you do deserve this.