It is interesting how "the other woman" isn't often jealous of the boyfriend's RL girlfriend, but afraid of other women online. I think why it's such a common issue is because those women are your true competition. He's already stepping out on his RL GF, so you know he chooses you emotionally over her. It's a very hard situation to be in, and very real. Online relationships are the true dating field these days, and the relationships and feelings, and most certainly the attachments are very real.
That being said, there is something wrong in your relationship and you're both picking up on it. It could be that you're no longer willing to ride in the back seat, which is natural. You may have insecurities that are inside of you as well. But what I would imagine the situation to be is that the course of your relationship has come to a turning point and needs to develop further and become more exclusive, or you're both facing the fact that it isn't' going to happen.
For you...if this is over, you need to mourn it like any other break-up: with gentleness on yourself, talking to friends, reflecting back on the good and the bad, and working towards acceptance. You do not want to stalk him - that will eat you alive. If it means not going to the site you met on, then let that be the truth for now. Resist the urge to stalk and to go back and forth.
You need to decide how much of a relationship you want in your life, and if this one is enough for you, or does it take just enough of your attention that it keeps you out of the dating pool. If it went on for 5 years like this: would it be enough? What if he marries his GF? What are YOUR limits. I think your time right now is best spent answering those questions so that you can go on with dignity and grace.
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