Thanks for the thanks (?).
I find that if I let my mind wander it tends to come back with a one true sentence that has a nice ring to it. But I usually tend to start by thinking “If I was telling this story to my best friend, my mum, my dad, my sister, how would I word it?”. Somewhere at some point in space and time I read that we should try to write just as we would tell a story to someone (be it a child or an adult). And it has helped a lot to overcome the “how do I start” problem.
I still struggle at times, mostly because I want it to be better, memorable, all that pressure as you say. But then the beauty of being able to edit and rewrite is there in all its glory and you can change it (even if you have already published, there is the possibility of a second edition, a clean up, a change…).
Keep writing! :)