Also notice that the OP said he has *tried* being the "nice guy", the "romantic", the "bad boy" etc...
IMO, that is a reason right there that sustaining a relationship is difficult. How can someone fall in love, and stay in love, with someone who is acting?
I think it is easier to develop a deeper, truer connection with someone if they fall in love with you, for being who you are and not who they want you to be. I admit, I did this too. It caused me a lot of pain down the road, as now I have a son from that girl and we ended up in a bad break up. That is truly a very hard situation to stomach, and one I could have avoided altogether if I had been myself. Either she would have left because I was not what she wanted, or she would have been in love with ME, and not a fictitious (sp?) me.