I had a girlfriend that I really loved. We would talk everyday, flirt everyday, and pretty much be your average cute long distance couple. I would also keep up with my girlfriend's best friend. I didn't really have a clue that I liked her, until I personally found out that she has a certain something that my girlfriend didn't: the need to simply be herself through and through.
Let me try to explain.
You see, this friend (let's call her Z) was more like the type of girl who didn't seem worried about getting a man. At times, Z was ragged and casual with her appearance, she was open with fart jokes and sticking straws up her nose, she never wore makeup, and she was much more of a rational thinker. She was a free spirit and much more mature in personality than G was. What I liked about her was that she seemed like the type of person who I could really have fun with or talk about something completely stupid with. With my girlfriend (let's call her G), I pretty much felt like I was dealing with your average teenage high school cliche. Sorta clingy, very insecure, unaware of how bi...er, mean she can truly be, etc.
G had a clue that I had a crush on her. It's safe to say she was jealous and didn't see the attraction. Despite that, I assured her that I wasn't going to do anything with her as long as we were together. Z was simply going to be our friend. And she definitely was. Whenever we both had problems with each other, we would talk to Z about it. Anytime we wanted to have some at-home fun, I talked to Z and G. It went pretty smoothly. But just because our relationship went smoothly didn't stop my occasional thinking about Z.
I never did make any move, until about a few months after G recklessly broke it off with me for good. After that, me and the "best friend" mutually (and slowly) clicked over love, jokes, how often I dreamed about her, and most importantly, visions over the ex-girlfriend. When we briefly got together after Z's divorce, I felt happy that the "girlfriend" broke up with me (note: she broke it with me due to a new local boyfriend, and an incredibly bitchy attitude) because it gave me a chance to really let the best friend know that she was the type of girl I liked all along. More or less, she was like a potential soulmate.
So, I felt guilty during the relationship for crushing on my then-girlfriend's best friend, but sometimes situations like that inspire you to have a choice: do you think you'll be much happier with the girl or the best friend?
And if you aren't happy with the girl, see what you can do about the other. Falling out of love is common, especially when you are amongst better company.