In love with girlfriends friend?

Before I start i just want to notify the reader that this is probably going to be a longish post. Recently i have started having feeling for my girlfriends friend. To make things more complicated, that person has been in my life for longer than she has been in my girlfriends life, we always got along, but then we got separated in highschool and didn't see or contact each other. Recently though we have been sharing a lot of good moments, have a few internal jokes, we know almost everything about our lives, she has helped me with almost every aspect of my life as I have helped her. I've been in a relationship with my girlfriend for 4 years and 2 months and she is the best person ever, loyal, loving, caring, responsible, nice to me, everything you could ask for. My question is, what should I do? How do i differentiate a temporary crisis/state with a general unhappiness in my relationship? Did the feelings for my/my girlfriends friend come because they had to come or did i do something to create that image in my mind that i shouldn't have done? Because i really love my girlfriend and before all of this i was ready to spend my life with her (changed the city where i study so i can see her more, changed my part time job so we could have more time together) and we've had nothing but good moments, the feeling of anxiety and depression and not knowing what to do have ultimately led to me not being able to eat for three days, i had to go see a doctor and now i am drinking like some liquified stuff with nutrients because i feel so bad that every time i try to eat i get a reflex. I don't know what to do because i know that relationships go through rough patches, i know that commitment includes sacrifices on every front, i know that being in a relationship is not all about the good stuff or about that initial crazy phase but about respect, tolerance and being best friends and sharing everything. I would appreciate any help with the matter and i hope my question/s were clear, if not i will be happy to elaborate cause i have to end this thing however it is possible, because it has started to affect my mental and recently even physical health. I also apologize for any vocabulary or grammatical errors since english is not my first language.