Boyfriend (28m) keeps her girl best friend (30f) a secret and they dated in the past

TL;DR My (25F) boyfriend (28M) has a girl best friend (30F) who I never met and they have been calling/chatting and hanging out during our whole relationship and after two years dating I went through his phone.

Hi! So I (25f) have been with my boyfriend (28m) for around bit more than 2 years. For context, the first year of us dating we lived in my hometown, then we were long distance for a year and a half and since this spring, we live in the same area. During long distance he lived half a year in his hometown (parents place) and then,since the past year, moved out of his home country for work (the area we currently live).

Ever since we started dating I’ve heard about his girl best friend and how they chatted and called often, which at the beginning I was happy because for me it is a good sign that my bf has female friends. It wasn’t until we had an argument 4 months in of us dating, that he confessed that he used to date casually this girl, and had sex bla bla all that, I guess if we never had any problem he would’ve never said.

All these years I’ve been curious about his best friend and he would tell me few times they hanged out to catch up and that she was very funny and always had a good time with her. The thing is I never met the girl, I have met most of his other friends (all male) but never the “best friend”. And he has met all of my closest friends.

Recently I had the opportunity to go through his phone and, even tho I know it’s crossing boundaries I just had to check what was going on with them, because whenever I asked about her he was always changing topics or, whenever I proposed to meet her, he said he would feel very uncomfortable.

So I went through their chats and omfg.

For context, the reason why he had to move back to his hometown to his parents, was because he was unemployed and wanted to save money. The period he was unemployed they were hanging out waaaaaay many more times than he would tell me. He would always say “oh I’m going for a beer or dinner with her”, but they also went to the beach, they hanged out at her place, they even hanged out in her car when it was rainy to have a “cozy dinner or beer”. Later he left to another country for work, and even then, they always text, call for 1/2/3h every few weeks and always when I am not there with him (I lived in another city). When the best friend texts him and I am visiting, my boyfriend tells her “sorry she’s here” as in I’ll text/call you when she leaves.

Whenever we had issues he would go to her to explain and seek advice, and same when the girl had issues she would call him and he would be the sweetest by hanging out, distracting her etc.

I had no idea they were so close and it makes me extremely uncomfortable, even I recall him explaining our first sexual times to her best friend who he also had sex with.

I honestly love my bf and I think, well thought, he was always transparent to me and relied on me emotionally, but what hurts is that when he was unemployed and getting rejected from all the offers and loosing hope while in his parents house, whom pressured him to get a job, I felt bad to not be near him to make him feel better while he was asking her to hang out and “very excited” to see his girl best friend whenever she had time, as he was always available. The thing is he relies heavily emotionally on her, like, every rough patch every bad feeling goes on me and on her, which made me feel confused because.

As I have moved to the same place he lives now, they haven’t been as active communicating as we basically see eachother multiple times a week, which makes me feel relieved but also nervous as, the second I leave for holidays he would call her and gets me thinking about what if he moves back to his hometown and what could happen there.

Also for context, my bf refers to himself demisexual, which means he only would have sex with people he has feeling with, my fear now is, what if he never lost those feelings.

So yeah I haven’t talked to him yet and I don’t know how to approach this situation or what I should do. I even feel bad for going through his phone but I am also grateful to come across this now and not in 10 years.