Hi all, I have to say that I experience this all the time.
Although this PhD is my fourth degree, Throughout my PhD, I've been plagued with thinking that they've made a mistake in giving me this well funded studentship. Everytime when my supervisors say something negative etc, I tend to think that their thinking that they should not have given me the studentship and should have given it to someone else who has a 1st class degree for example, or who is far more brighter than me.
Although I'm happy to say, these kind of thoughts have diminshed significantly now I'm approaching the last 4 months left of my registration period. Each time when I've challenged my supervisors suggestions, or contributed to other reports (and my ideas have been integrated into their governmental reports), or pointed out something that my supervisors have missed out on, or my PhD work ideas and questions has made them think about the bigger project; my confidence has slowly started to increase.
I do have the occasional day where I'm thinking 'I really don't want to do this anymore', but when I've found something that is highly significant and obtain praise and acknowledgement from senior academics (Professors etc) I start to feel a little better about things! Rollercoaster of emotions though I have to say!
But yes, imposter syndrome is definitely applicable to me!