First, our society now is (IMHO) suffused with despair and its second order effects, such as anger, on an extraordinary level. I stopped reading/watching/listening to that stuff a couple years ago, unless I absolutely had to (99% reduction) and, it was a gradual process, but I not only left it behind but gained perspective on it - what once seemed overwhelming is now just a (dangerous) absurdity. Despite the presumption now that humanity is doomed to corruption and despair, there's generations of more positive stuff to read/view/etc from, you know, about 4-5 years ago - crazy things like optimism, belief in the possibilities, hope, justice, freedom, etc.
Second, just promise yourself no more rants, hyperbole, etc. Promise you will write constructive things. It will take some effort at first but you'll find there is far more to say (and that the rants have no value). Any time you find yourself in rant mode, stop, which leads to ...
Third, find healthy outlets for yourself, activities that, when you are finished, you feel better and healthier than when you started. (Otherwise, it's not much of an outlet, is it? But we all pickup unhealthy ones before we know better.) Exercise is a simple one.
Most importantly, by far: If you don't understand your emotions and have compassion for them, they will own you. If you find yourself with any unrelenting emotion, figure out what you are really feeling negative (or whatever) about - you probably are just redirecting that toward an easy outlet (someone on the Internet) rather than the real target that you don't want to face.