I understand your struggle very well, I too, went away from steemit for a while, because I felt that my content and my ideas weren't good enough for this platform, because I kept seeing such great posts and I felt that what I had to give just wasn't cutting it, but I decided to give it a go once more, because our minds are our worst enemies.
My advice would be to keep practising and writing, English isn't my first language either, but I am always writing and reading (even if it's just small meaningless stuff) because I just got myself used to it, I can't stop hehe.
Practice with yourself, write for yourself and don't compare your writing with others, compare it to old stuff that you've written, that way if you make progress it will show somehow and you'll realize that you're improving. Keep reading, and also try to find at least 2 or 3 new words each day, and try to incorporate them into your writing that day, so you keep expanding your vocabulary. It isn't something that happens overnight, and it's okay if the only thing you do on a week is learn one word.
"Progress is progress no matter how small."