I definitely have it, or else I wouldn't be a USA freelancer w/ 5+ years php experience (laravel + vue = current stack), but only charge $40 an hour as a freelancer... See, I KNOW I have it, I KNOW I'm worth more, but when I get into negotiations on rates, I freeze up and low ball to get the client rather than just spit out $100/hour take it or leave it... I'm in therapy for anxiety/depression though and focus/concen…
It sounds to me as though you struggle with negotiation due to a lack of confidence. That's not ideal but it can be remedied, and it's a different (better, in my opinion) issue to have. On the other hand I don't really think you can have imposter syndrome while being self-aware about having it. Imposter syndrome is a thought process, not a behavioral pattern. Understanding that you're worth more than what you're charging means you're aware you could improve in negotiation. How would you be aware that you're worth more while also being aware that you believe that you're not worth more?
On a more practical note: I used to be a consultant. I initially struggled with negotiation as well. My advice to you is to practice being flippant with the numbers you provide. I know that sounds cavalier and it's easier said than done, but it can be a very empowering (albeit nerve-wracking) experience to learn how to give ridiculous numbers out. You might be surprised how often they're accepted.