In love but confused

Hi,

So I'm here because I really need some advice/comments on something that has been constantly on my mind the past few weeks. I guess I should start off with some background, I'm 22 and I have been dating my girlfriend for the past 9 months. She is really sweet and amazing and I'm very much in love with her. She's two years younger than me, which isn't that big of a difference, however I feel like there's a serious gap in our maturity levels. I often feel like I have to "mother" her, insisting she go to doctor's appointments, telling her to drink water when she drinks way too much, and waking her up for her classes/appointments etc. when I have to drive her (she is impossible to wake up and often causes us to be late.) Nevertheless, she is the sweetest girl I've ever met, she makes me happy and I know I mean the world to her (as she does to me). I should also point out that I'm the third or fourth serious lesbian relationship she's been in (and she's hooked up with many more) where she is the first person I've ever dated and the first girl I ever kissed. I often wonder if I am doing myself a disservice by becoming so serious with the first person I've ever had romantic feelings for, but I know she loves me more than anyone she's every been with, and I love her, so I usually reassure myself I don't need to date anyone else.

So, here's where the main part of the story comes in. After I started dating my girlfriend, I met her friends, all of whom I absolutely adore, they are a wonderful group of people. One girl in particular I was very much attracted to (physically but most importantly her personality). My girlfriend knows I'm attracted to her friend (as does the friend), but her friend is "straight" with the exception she sometimes makes out with girls and on occasionally sees a girl she is attracted to, and claims she would do more than make out with. She also has said she wishes she could be more attracted to girls because boys are stupid. I've always made jokes with my girlfriend about being attracted to the friend, which she usually doesn't mind but I know when to lay off if my gf seems to be getting annoyed.

For the past several months, I haven't thought much of the friend (I'm going to refer to her as S from now on) mostly because it was summer so I didn't see her much. Now I'm up at grad school several hours away and my girlfriend is at her school with all her friends. Starting from the first week she moved back to school, she said all her friends missed me and that S said she loves me. S and I have been joking/flirting through my girlfriend (i.e. S will ask when I'm coming to visit her and that she's going to take me out to dinner, which my gf relays to me through text, I text my gf back and tell S I'll visit her soon and buy her some nice wine, etc).

Normally I would think nothing of this, S and I have very compatible senses of humor, which is why I've grown closest to her out of all my gf's friends. However, this has continued over the past three weeks, with her sending me texts from my gf's phone saying 'I love you with all my heart, please run away with me.' Which again, is said in a joking way since my gf is right there, but at the same time, if this is just a running joke I feel like it has carried on for much longer than it would have if S has no real attraction to me. I'm wondering if because S has expressed attraction to girls on the rare occasion before, if she does have some sort of feelings for me. (She has seemed jealous on a few occasions, once when I told another female friend of my gf that she was hilarious, S replied with "But you still think I'm the funniest, right?", and she also asked my gf and I during a game of truth or dare who we would choose in a threesome, and when my gf picked a different friend, S looked pissed). Also my gf showed S a snapchat from me the other day, and S commented on how good my boobs looked. Just wanted to throw that out there.

So basically S and I have been joke/flirting through my gf for the past few weeks. Honestly, my problem is just that I am very attracted to S in many ways. I always valued sense of humor in people I'm attracted to, and I'm very attracted to her dry humor (something my gf doesn't really have aside from the occasional pun or joke). I also really respect S's maturity as it is very similar to my own. I've always told myself my gf is just immature because she's younger than me, but lately she has been doing a lot of things behind my back, especially when she's wasted. A couple weeks ago she went skinny dipping with friends and a couple strangers; S opted out of doing it and I really respected her for being the only one not to, because I would have done the same thing. My gf only tells me me about these things after the fact because she knows I would try and talk her out of it (I tried to talk her out of skinny dipping because she was wasted, she said she wouldn't, but the next day told me she did). Her immaturity has always bothered me, but her adorable and sweet personality always made me overlook it.

I really do love my girlfriend, but I feel like if I really did have a chance with S, I could be happier. S has the ideal personality I've always looked for in a partner, and I can't stop thinking about her. Please let me know what you think about this situation, if I'm just being selfish and unfair to my gf or if I should try and see if S does have a real attraction to me. Also, this isn't about looks either, my gf and S are about equally attractive but I am very drawn to S's humor, maturity, and overall personality. I don't want to hurt anyone, although it may be inevitable, but I also feel like I shouldn't stay with someone if there's someone better for me just because I want to spare heartbreak.