Assalaamu alaikum,
Mmm, if I understand correctly, you have a girlfriend in real life, but feel the urge to go online and chat with other women?
It seems to me that the internet is easily addictive, because it instantly palliates various needs we have. Perhaps exploring what that need is, will help you break this addiction?
(mildly) If you have a girlfriend and you are attached to one another, why haven't you married? Perhaps you could look at yourself and ask yourself why you are not committing? Are you afraid of the responsibilities of married life, for instance? Perhaps your online adventures are an expression of this fear?
I don't know, because I don't know enough about you. But trying to understand your deeper motives might help you change your behaviour, I think.
Incidentally, going online and flirting is a form of cheating, in my opinion. You are sharing something of yourself with a woman other than your wife, and you are perhaps leading other women to believe that you are available for marriage. This is not really honest, is it? And you could hurt someone.
Anyway, kudos to you for admitting you have a problem. It's the first step to recovery.
May Allah, the Victory Giver, Help you in your struggle.