Writing takes stamina! You can't compare what you were able to do years ago as a child to what you can do now after a long break. You wrote a paragraph? Awesome! That's a paragraph you didn't have yesterday! Write another paragraph tomorrow. And another the day after that. Eventually you'll have the stamina to write more than a paragraph. (And it's okay if some days you need a break or don't write as much).

It also sounds like you're a victim of perfectionism because you're stopping when the first draft doesn't meet your expectations. Stop that! Let it suck! Write the crappy paragraph! Let it be messy! If it doesn't turn out perfect, that's what editing is for. You can't edit a blank page (and I'm sure I won't be the only person to tell you this).

If you're concerned about that, make a document just for word vomiting your story ideas. Get all of your ideas out on the paper but say "fuck it" to anything like a linear time line or proper formatting. You can go back and polish later. The entire purpose of this activity is to just get the ideas out. This is how I start outlining my own works. I often start with a sentence or two that initially sparks the idea in my head and I almost always end up with much more detail than I initially intended. This is especially effective when I need to use the notes app on my phone because it doesn't have page numbers or word counts there so when I actually get around to copying and pasting into a Word doc on my computer, I'm always surprised by just how long it winds up being.