Starting my Common App essay was such a nightmare at first. I felt this pressure to have a killer opening line, but that just made me freeze up. What actually worked for me was to just write out my thoughts as they came, like I was telling a story to my friend.
Once I had a plain version down, I went back and looked for the most powerful moments in the essay and brought those to the front. It made the whole process less stressful and honestly more enjoyable.
So, if you're feeling stuck, just get your ideas down without worrying about perfectly crafting the opening. You can always edit later!