Gripping a pick 'properly' kills my hands, holding it like James Hetfield is the only techniue that doesn't hurt, is this just me?

I should preface this by saying I do have chronic fatigue and it's likely this is what causes pain, I grew up picking with the thumb and first two fingers but was told to stop and try picking 'properly'.

Ever since starting to pick 'properly' my technique is awful, my thumb starts to kill and it's like I'm getting worse, is it just me??

(Obviously it's not just me, considering it was popularised by hetfield, ykwim)