Help?? pretend to be a man online?

OK so first off im a girl i just made a quicky account to ask this question. so im bi and i think since 08 iv been going onto websites and pretend to be aguy?? what im asking for im not really sure. why do i do this? do i yearn to be a man? ik iv had feelings like sex change wise but im not sure if thats for me. i go onto these sites and pretend to be a guy maybe because it fufills my fantasy of actually being a man?? illhave like a gf on the site and they have no idea im rlly a girl. i feel really really guilty about it but i cant seem to stop. im just afraid someone will figure out the truth. honest answers only please nothing negative. and ik that its wrong what im doing so plz nothing mean