I don't know... that's up to you. You have to be really 'optimistic'... but really, if it were me, I would probably give it up or wait until they break up. I wouldn't go after someone when they're with someone else (as cybog said, imagine yourself as the girlfriend and some other girl trying to take him away). Doing stuff like that isn't very nice, in my opinion. I'd say to just keep being yourself. If you're being yourself in front of him now, just keep doing that. Just remember if he does have a girlfriend to not get your hopes up so high so soon. That'll just make you really depressed.

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If a dream comes true... then is it still a dream?