Oh, the impostor syndrome...
It's one of my continuous struggles. I do have that cruel voice many of you probably know... I'm not an artist, so it doesn't talk about art, but it talks instead about my personal relationships. In particular, it's about relationships with women and it comes when I'm having doubts about something, or something doesn't go well or perfect even. It appears as an intrusive thought which goes like "How I even dared to talk that woman in the first place?! I'm a horrible person and she must be ashamed of herself because I talked to her and disturbed her." or "I'm so ugly/I'm mentally ill and it's so obvious, so of course she must hate me.".
What I'm trying to do and which works sometimes (sometimes not, unfortunately) is that I try to analyze rationally what's going on and whether what I'm thinking has anything related to reality. In most cases, it becomes obvious that I'm just blowing things out of proportion, like focusing on some little and insignificant details, or worrying about something that didn't happen and may never happen at all.
I also found that asking for constructive feedback, sometimes just even talking about the situation, can help. Usually the other person doesn't have all those negative core beliefs, so she can actually explain the situation from an objective point of view.
In your case, maybe you can ask someone to evaluate your artwork and help you to establish an adequate pricing? Doesn't have to be an art expert, just someone who you can show your art and who can talk to you gently about it.