I try to set myself time limits for my forum use, and set an alarm - and decide beforehand what I'm going to do when the time runs out. I allowed myself to finish a post that I'm in the middle of writing when the alarm goes off, but then I have to log out and do something away from the PC.
It certainly doesn't always work - I know just what you mean about getting sucked in. But if you limit your time, and go on the sites that give you the most enjoyment first, you can tell yourself "well, just isn't time for that horrible forum, I've got something better lined up to do."
It's a lot easier to say than to do, of course. But at least you have realised now that there are some forums you are better off avoiding. The first, and hardest, step is to admit that there is a problem - so allow yourself to feel some pride in that, it's an achievement in itself.
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When you are fighting an invisible monster, first throw a bucket of paint over it.