I don’t know how to do my work well

I have these projects at work to test these websites and I keep thinking that I don’t know what is supppsed to be on these websites so that I can test them. It’s keeping me spinning in circles and I feel anxious and dread when I think about completing the tasks. I feel like there’s always something that I don’t know about them and so I doubt my work and whether I’ve done a good job or whether it’s even done. I have asked questions Of my colleagues to try to find out more and it’s not helping at all. I want to feel confident and capable about these tasks and right now I feel dread and overwhelm and doubt about them. I don’t know how to stop this feeling that I don’t know what’s going on with these projects.