I think you're accidentally clicking on the "new topic" button instead of the "post reply" button.
"New topic" will start a new thread (like the one you've made here); "post reply" will let you add to a thread, as I've done here.
A good rule of thumb that if you've been able to give a title to your reply (like your "yes, I've been there"), you've probably started a new thread by mistake. If that happens, click on the "back" button on your browser, then click on "post reply". You'll get a box where you can type your reply, and then you can click on "submit" when you're done.
You can also type in the "quick reply" box beneath the thread you want to contribute to, and then press "submit". That might be the easiest way for you to participate in the discussions until you get used to the layout of these forums.
Co-Moderator Crohn's Forum.