SUBREDDIT: r/relationships
TITLE: I (28,M) and my gf (26,F) need advice about how to handle my female best friend...
POST: Been dating 9 months. 

My best friend is a female who has been dating the same guy for about a year and a half. We met on a dating website, went on a couple dates, made out once, but both decided we are better as friends and that is how it has been for two years. I am friends with her boyfriend, no issues there.

I jokingly refer to her as my life coach as she often helps me think through work, relationship, friendship problems and she's good at it as she has a degree in psychology. 

Started dating my current gf and my best friend is excited to meet her, get to know her, etc. All good signs. My gf had some insecurities from past relationships.

My gf went through my tablet at 2 months into our relationship and found things from past relationships that upset her which caused her further distrust. At that point I said she can look through whatever whenever.

At 8 months my best friend and my gf do not get along. My best friend doesn't think she is right for me and thinks she is controlling and is pulling me away from my friends. My girlfriend has read texts from her saying these type of things and we argue about how I need to just cut my best friend out of my life.

Recently my gf has been distant and pulled away. I got sick of arguing about the things my best friend may text so I have deleted rude things she says which my gf then finds out and is upset. 

My gf solution is to basically cut her out, if she texts just be short, brief and we will not hang out with her and her boyfriend anymore. I realize I made a mistake by deleting these items but I for the most part just get tired of arguing about it... Advice?
TL;DR: 
my gf and my best friend don't get along. My gf wants me to stop interaction, I don't know how to create a win win? I have deleted texts that may upset my gf...