Imposter syndrome is a thinking trap. A couple of things you can do to help:Try to separate out 'ruminating' from 'thinking'. What's the difference? For our purposes 'thinking' has a fixed outcome and an end point. Trying to solve a coding problem. Working out how to make dinner. Calculating your taxes. When you reach your goal, you're done and you stop. 'Ruminating' has no end point. There is not end or action associated with it. The tricky part is that it often masquerades as thinking, so you feel like you're solving a problem.
For example, you do a job interview and you go over and over what happened in your mind. "Maybe I should have answered this differently". "Maybe I should've prepared some more of these questions". Of course, you can't change what happened in the past. You're just rolling the ideas around in your head and probably making yourself feel worse and worse. Rumination can be focused on the past, the future or even some hypothetical, imaginary situation ("What if I lost my job", "What if my house burnt down"). Again, actual preparation (Saving an emergency fund. Getting insurance) has an action associated, but rumination never ends, it just keeps going around in your head.
The other thing is to keep an accurate record of your performance. This will be different for everyone, and varies a lot depending on the job. The key thing is to make the record as close in time to the action as possible. For example, you feel like your pull requests aren't as good as other people's. Don't wait until the end of the week and then reflect on the quality of your work. Instead, every time your make a pull request, write down an accurate, objective assessment of the quality.
People who suffer from imposter syndrome tend to forget their wins and remember their losses again and again (there's that rumination!). By having an accurate record that you made yourself you can cut through this and show to yourself your true performance.