I have fantasies about my gfs best friend

I(m22) have been holding this in for a while but yes I am very very attracted to my gfs(both f20) best friend. It’s very much sexual but sometimes I do fantasize about us being together. Believe it or not I didn’t always feel like that. When I first got with my girlfriend I thought her best friend was intimidating and honestly kinda rude. I didn’t like her at all. There was just some things she would do that would just piss me off/rub me the wrong way and I very much felt like I hated her. Ironically I think that’s what sparked the feelings I have for her. I remember one time she did something that I thought was very disrespectful and I just randomly thought “I need to hate fuck her” of course the thought caught me off guard cause in all honesty I’ve never really thought that way towards anyone. It was like an intrusive thought. It came and then the thought left my mind. But then I thought about it again and I thought about the actual act. I thought about it again and again but then I just brushed it off. Just continued not paying no mind to her. She honestly is cool and I didn’t have a problem with her after the incident that I thought was disrespectful. We were cool. We all hung out (me, gf, best friend, plus another friend) and it was cool. But idk why on a random day she posted a pic on her Instagram story and my jaw dropped and pants raised no bs. It was this sexy pic of her in a green outfit. I’ve never seen the color green look better on someone in my life. The outfit made all her curves so defiant. She just looked perfect. That pic unlocked something inside me and that’s when I realized oh yeah I want to get in her pants. I felt so guilty because my gf and I are doing so well and we’ve been going strong and here I am thinking about fucking her friend. I even touched myself to the pic. I know in some relationships partners watch porn on their own time but I felt that this was different because it’s someone so personal to my gf. But I just couldn’t get the feeling out of my head. My gf and I are great. I love her with my life and soul and I would never do anything to ruin our relationship. These are simple just thoughts that I’ve had. And it’s not that my gf isn’t hot. She’s drop dead gorgeous. You know the saying “I ain’t ever seen two pretty best friends” throw that shit out the window with these two women. I guess to me it’s just something about her best friend being unattainable, the forbidden fruit. Also I lost my virginity to my gf and I’ve only been with her so it’s also the curiosity of other women I guess. It’s just a lot of feelings and I try to just burry it but I can’t help but look at her best friends instagram and save some pics and hid them. I treat my gf with all the love in the world and we are in a very healthy relationship. I feel like a piece of shit because of these fantasies. The feelings go deeper but this confession is already long. I will NEVER peruse her friend but I just have these thoughts in my head about her. This is the first time I’m speaking about this to anyone and it feels a little better letting it all out. I just don’t want this to ruin my relationship because I want to be with my girlfriend and she’s been there through some tough moments for me and I for her. I just hate these thoughts and wish they will go away cause I fear they won’t do nothing but harm the relationship in the long run.