You must find a balance between mindfulness and thoughtlessness. Think too much and your words will have no soul, or you won't write at all. Feel too much and your words will have no structure or be unreadable. You must find a balance between the two, where you words come out as you envision. This is also something I struggled with when I first started writing.
I also had the thought that my understanding of the English language wasn't good enough, so I read half a dozen books - and another on grammar - and tried my hand at writing again. You don't really say why you can't write, you simply say you can't; I find that quite annoying. If you're going to ask for assistance then the least you could do is state your problem clearly, or give clues if you don't know it.
Also try and aim for around 500 words at the start and eventually work your way up to 1,000, to 2,000, to 3,000 and so on. It's hard for a work to feel finished if you don't already have a vision for how it'll start and end. And it's much easier to imagine a larger piece if you work up to it. I think it's like reading a book. Reading a 1,000 page book when you've only ever read 200 page ones seems nearly impossible, but if you work your way up, it's much more manageable.
My fix for balancing thoughtfulness and mindlessness so I could constantly write in that flow state, where everything just felt right, was to write when I was sleepy, so either when I woke up or was about to sleep. At first I couldn't do it at will, but I eventually remembered and emulated the feeling, so I could write even when I wasn't sleepy. For me, It's kind of hard to remember what the flow state feels like, since an hour passes in an instant, but it's like a churning fire in your chest. It might be different for other people though.
I think a lot of writers are boring because they don't write while they're feeling it/in the flow state; or they don't think one exists or is obtainable.
Am I missing anything? No, I think-I think that's it. Find balance. Believe in yourself, but not too much. Work hard.
You say you've written before, but finding balance was literally the first thing I did to start writing. I think it's like riding a bike; once it clicks it stays, a sort of muscle memory.
Uhhh, and don't forget your toothbrush!