Insecurity can ruin it, yes. Once you start overthinking what you do, you ruin it. Best way is to close eyes and just let it go.
Aaaand also some people just don’t know how to do it, though it seems quite easy. But no, not all people do it well. Some use tip, some entire surface, some are fast, some are slow, some can’t find the spot, some don’t listen to their partner and then change way they do it so the partner looses the feeling, some use hand also, some don’t etc… It can be actually very complicated once you start analyzing it. There are ways to do it badly. And if one does it bad, tell them. Maybe you actually like it.
And if you don’t, that’s okay to. Not everyone likes chocolate ice-cream too.