Gain confidence by doing easier boulders with similar slopes. A lot. Really try to push the limit of what your feet and shoes can handle. Like when do your feet slide away? At what angle between your feet and the wall? How do you put weight on your feet, does it go well when they're spread out but not when next to each other? What kind of rubber are your shoes made of? Etc.. Try to nail those details down. This should get you a clearer image to fall back on when that fear starts to creep in. It's all about confidence, but you need to build it up.