I have had with one woman. Help!?

Guys, I have been in a 3.5 year relationship with a girl. I am in my mid 20s and I have had with only her in my life. Hooked up with 3 or 4 other girls but no . My gf could be a bikini model (seriously, she has had offers) and is very seductive. I am having trouble ending the relationship I think because I am scared to lose the and not be able to find another girlfriend as attractive. Everything is pretty much screwed up in our relationship and I am not very happy. Very needy, lots of problems, past issues, physical problems, and kind of drains me. Its hard to leave her for some reason. Anyone else had a similar problem with the first girl they have been with. Do you just think that they are the greatest until you have been with multiple woman? I am a good looking sucessful guy so I will be able to find other woman. Any suggestions?