Well, maybe some clarity, seeming as I have none as of right now.
Clarity about what? Your own desires? That's not something any of us will be able to give you.
There are many possible resolutions here. I will assume for purposes of pronouns that the "friend" is female.
1) You ignore your feelings and stay with the girlfriend.
- a) Everyone is happy with this
- b) The friend is silently wishing to be with you but not saying anything because you have a girlfriend. You're unhappy because you want the friend. The friend is unhappy because she wants you, then finds a boyfriend. You respond to her getting a boyfriend by internally giving up on your desires for her and trying to make the relationship with the girlfriend fill your needs. But it doesn't because you wanted qualities from both girls, so expecting what you were idealizing from the friend from the girlfriend doesn't work and the relationship falls apart. But now the friend has a boyfriend and you're now in her position of the person you want to be with is involved with somebody.
- c) The girlfriend is happy with this and the friend is completely unaware that you were ever unattracted to her, continues to spend time with you which drives a stake into your heart at how much you want her and how close she is but you can't have her.
2) You give up the girlfriend to be with the friend.
- a) It works, everyone is happy.
- b) The friend isn't interested, but you already gave up the girlfriend so you end up alone. But at least you have a clear conscience because you didn't cheat on anyone.
- c) The girlfriend is clingy and doesn't give up and destroys your relationship with the friend
- d) The girlfriend is clingy and doesn't give up, and the friend accept this and you end up with an official girlfriend and an unofficial one.
3) You covertly attempt to liaison with the friend without the girlfriend knowing
- a) The girlfriend finds out and leaves you then spreads nasty rumors about what a jerk you are. The friend thinks it's slimy and stops talking to you. You lose them both.
- b) The friend isn't interested and the girlfriend finds out, gets clingy and jealous, and stays with you and makes your life miserable.
- c) The friend isn't interested but the girlfriend doesn't find out, and you learn to become a sneaky cheater, probably carrying that habit into your next relationship where it will either bring you much joy and stress, or destroy you.
- d) The friend is interested and the girlfriend isn't smart enough to realize that you're cheating on her. Everyone knows, lots of people try to tell her, but she never really gets it and continues being a good, dependable girlfriend for you. Eventually you lose all respect for her, leave her and she spends the rest of her life in misery and self-doubt because she did everything she could to make you happy and ended up being abandoned with nothing because of it.
- e) You succeed in making the friend your mistress and the girlfriend never finds out. You have a girlfriend and a mistress, and yet somehow become a karma houdini and it all works out.
- f) They find out but they're ok with it, you have two girlfriends.
4) You tell the girlfriend that you want to keep her, but also want to start seeing the friend.
- a) It works, everyone is happy.
- b) It works, but everyone is unhappy and resentful.
- c) The friend is interested but the girlfriend gets angry and leaves you
- d) The girlfriend is willing, but the friend isn't. The status quo is more or less maintained, but you have some closure and a girlfriend you feel you can trust with the truth.
- e) Neither of them are willing, but because you're honest about it the girlfriend doesn't get angry and you keep her.
- f) Neither of them are willing, and the attempt causes the relationship with the girlfriend to fall apart
- g) The girlfriend is interested, but only if she get to be with other guys. You agree, it works and you have an open relationship.
- h) The girlfriend is interested but only if she get to be with other guys. That was totally not what you wanted at all, you're totally not comfortable with it, but the genie has been released and it spirals out of your control.
5) You tell the girlfriend openly and honestly that since you started dating her you've suddenly become attracted to a friend who you weren't previously attracted to, and then you invite her to participate in the process of deciding what to do about it.
- a) She freaks out and leaves you. You are then free to pursue the friend with a clear conscience.
- b) She's vague and unclear on how she feel about the situation, leaving you uncertain how to proceed, but at least knowing that you weren't deceptive about the situation.
- c) She is in such shock that you were actually honest about it that she doesn't know how to react, it doesn't trigger any "cheater" vibes because you told her and that's not what cheaters do, so you are able to pursue the friend while keeping the girlfriend. She i aware of this, but doesn't really know how to feel and for an uncertain time you have two girlfriends while everyone sorts out their feelings.
- d) She' totally ok with it and you have two girlfriends.
- e) She's either ok with it, or not strongly opposed, but you open dialogue with the friend and she's not interested, leaving you with a girlfriend you feel you can be open and honest with.
- f) She understands that there's something peculiar going on because you didn't start feeling attraction to the friend until you started dating her, she is curious about this and the two of you explore your psychology together openly and honestly, resulting in only one girlfriend but knowing more about yourself, and a girlfriend who knows you in ways few people are honest enough to share with each other.
I'm sure I've missed a few possibilities, but I think the above probably covers are good portion of how this could work out. I suggest either going through the list and picking a resolution you like more than the others, and trying to make it happen, or making your own list and doing same.