I write for another website, and I do so anonymously in order to talk about abuse and other things that I have been through and that I may not want my friends and family to know, or comment about. It is difficult, it is almost like witness protection in a way. You have to separate yourself emotionally from what it is you are writing. Our emotional state comes across in our writing, and is often a dead giveaway as to who we are. Also choosing a name that has nothing to do with who you are, or the things you like or where you live in real life is key to protecting your actual identity for a situation like this. Avoid using any phrases or terms that you tend to use when interacting with friends and family ~ especially any cutesy things you or your family often use. As an example my family says 'Love you bunches' rather than the typical 'I love you'. Keeping to mainstream word usage, and avoiding any signature phrases or words will help you retain ambiguity.
I do find it helpful to have a friend or two that knows my anonymous identity. It is helpful to have at least one person that you can discuss it with, making it easier to keep the secret from others (even if it is just an online friend, and no one you interact with in person. Hope that helps, and have a great journey writing anonymously!