Writing is difficult. Some people devote their entire lives to writing, and many of them admit they still don’t have it quite right.
Like any other skill, writing takes practice. You’re not going to be good when you start. You may not even be good after a lot of practice. That’s okay. Remember, every time you string some words together you’re getting a little bit better. That’s what counts.
There will always be better writers, and there will always be better copy. It’s useful to look at great writing for inspiration and ideas, but don’t demand too much of yourself.
If you can’t assemble the masterful sentence your mind envisions, take solace in the fact that you are not alone. Also, and this is key, what you write doesn’t have to be perfect, just good enough. Are you getting your message across? Did you find and remove the typos and other technical errors? Good enough!
Put in the practice and give yourself some grace on the hard days. Stretch yourself, strive to write well, but be okay with words that are good enough.