So i have this one friend that i knew 2 yeara ago. me and her became super close, i tell her everything and vice versa. Before i got into a relationship with my gf, i used to kinda like my friend but i didnt do anything and after i got into a relationship with my gf, i totally dropped the feeling for my friend. 

fast forward a year, 
my gf knew that i used to like my friend and knowing that me and my friend is super close, she kinda feels threatened and she thinks its possible i would catch feelings for my friend again. i feel like that is total crap because im not the type of person to love/like another person if im already in a relationship. so i told her its nothing and at one point me and my gf got into a fight that lasted for 2-3 weeks. and in the end, she made me choose between her or my friend. now i dont want to leave my friend because i dont really trust people easily and my friend has always help me, how do i just leave the person that has been with me through thick and thin? but at the same point, i dont wanna lose my gf cause im too in love with her and shes actually the most decent person ive ever date and weve already made plans to marry her, have kids, etc even our parents know about us. so i told my gf that i chose her and that i would stop being friends with the said friend but in reality, i still talk with my friend behind her back, i archive my conv with my friend everytime me and my gf go on a date. she was still hurt and said if i became close with her again, she'd leave me. but after 2 weeks she was still okay but she does remember this once in a while and that ruint her mood.

fast forward another year, (this year)
i fucked up and forgot to archive my message with my friend during a date. my gf played with my phone and you guessed it, she found the chat. she dropped the phone, look at me and immedeately ran away. i didnt chase her, i couldnt chase her. after she ran away i fell down and cried. in public. yeah that was embarrassing but i didnt think anything at all. i remembered she said she would leave me if i did it again and then i fucked shit up and got caught. i called my friend and she calmed me down and helped me calm my gf down. (i forgot to tell you, my friend is my gf's bestfriend lol) my gf became cold with me, she replied super short to all of my messages and she got mad at me but one day i force myself to meet her face to face to talk about it and she decides to give me a last chance, as long as this dont happen again. i was super happy that happened and all that but the thing is, i will never leave my friend and i continue on doing the same thing. i mean i just cant leave my friend cause she's the best friend ive ever had, she's been with me for so long now and i just cant leave the best friend ive ever had. 

So now i feel kinda bad for doing this for my gf but at the same time i feel like she shouldn't do this to me.

I need your guys' help on this one. AITA?