Hi, Nuinip. Online, a person can be whomever you want her to be, an idealized image and not a real human being with real problems. You don't have to know her as a person, you can pick and choose from clues and cues she puts out there, and create an ideal dream woman in your head. It might help you to disengage from your fantasy if you can understand that what you have fallen for is an ideal image is one you made up yourself. She did her best to look good online, as we all do, but you filled in all the blanks with what you want a romantic partner to be. It might help you to understand this when you wake up from your dream -- it's easy to fall in love with a fantasy you made up yourself. It's just not real. Let her go off to her planned marriage, and don't be sad. You didn't really miss out on her, you just had a fantasy that you developed yourself.
You should ask yourself how you would feel if you were about to marry a woman whom you found out was chatting with another man on Facebook. Put yourself in her fiance's shoes and think about how upset you'd be. Then leave this girl alone and find someone who doesn't already have a boyfriend or a fiance. If you continue to pursue her, knowing that she is about to be married to someone else, that makes you look like a really bad person. Don't do it!
For me, besides all the stuff that it is hard to have a 'real' romantic relationship that has been strictly online . . . she's about to get married to a 'rich' man. Two things . . . one, you now know her character. She will marry a man for money while carrying on with other people behind their back. That's not your dream girl. And second, she's getting married so not dating eligible. She's already taken and you need to leave her alone.
Do you have other means to meet people?