Disgusted With Myself

Me and my gf have been dating for about 11 months now. I lost my virginity to her just this past March and it was everything I ever wanted but the problem is I've been thinking about how sex might be with other girls. Even worse is I have girls offering to sleep with me lately with no strings attached. Even my best friend says she wants to see what sex with me would be like and I've never ever cheated but it's proving harder and harder each day to remain faithful when I'm bombarded by free sex.....I just don't know what to do.