Honestly mate, I'm like you. I used to post on forums quite a bit in past (quite a few years ago now) and then hardly posted at all. This forum is the only website I am pretty much active on these days when it comes to posting stuff. I do still get anxiety, and I still feel nervous about stuff, but things to gradually become easier. Back in March 2015 (my join date to this website), it took me a few weeks to even sign up, and I felt really nervous about it. After a few weeks, it got easier, then easier, then easier. As I said I am still a bit nervous about stuff, but nowhere near as bad as I became. I was lurking from the end of 2013, and I joined over a year later.
Just post some easy going stuff to begin with, it may help. For example look in the Just For Fun section and post in the random threads. Even though at the minute I pretty much only live in that section (lol), it's mainly just because I am just a bit depressed and not knowing what to do, rather than anxiety about posting in other sections of the forum.
Take it slow, and only post if you really want to, and if it helps, try and twist it a little bit, so you don't have to talk about yourself so much, like "when I went out today" could be "when people go out sometimes, they may..." if that makes sense?