I find that what I personally struggle with in writing is feeling like my opinions are stupid or that what I'm saying just isn't worth, probably what you said about second guessing. In these situations I just try to not care for a little bit and write even if it feels ridiculous and then read it after a while to see if it makes any sense at all. Probably not a very helpful advice since I came to this post to see if I could find any help from the comments as well :D