The trick, I've learned, is to write about things I like and spend very little time discussing the things that I don't like. If I catch myself writing an incredibly negative post, it's usually because I'm wrapped up in a lot of emotion. Maybe somebody hurt my feelings at work or rehearsal and I wanted to write about how that person and everything they do is bad and is hurting the fabric of society. Then after I've written it, I read it back and it all seems a little silly.
If I write about my own experiences and things that have made improvisation fun for me, then who is anybody else to tell me that I'm wrong? If I spend posts upon posts being a positive beam of sunshine and someone comes in and calls me a dummy for enjoying things, that person is clearly just an a-hole. If you write about what you love, kind people will respect that (unless what you love is kicking puppies or something). The added bonus is that if you spend your time thinking and talking about things that you like a whole lot, you're going to feel a whole lot happier as a result.
Go ahead and get angry and write posts seething with vitriol. Get it all worked out of your system, then re-read that post and delete it immediately. By spouting negativity you are inviting other people to spout it right back at you. Granted, some things are worth posting takedowns of: puppy kickers, MRAs, people who order a diet coke and a water at a restaurant.
If you want to feel better and not worry about people telling you how awful and dumb you are, you should start by not talking about how awful and dumb everybody else is.