I need relationship advice

As I can’t talk to my friends about this as they aren’t the type of friends who would give me advice or help me I really need your help and advice please..

So I recently found out my boyfriend has been talking to other girls behind my back through Instagram but I know one of the girls was a friend but I know she really likes him a lot with makes me so jealous as she has sent him photos in her underwear before and this morning around 4:30 I had anxiety attack I notice my boyfriend was awake but sometimes that’s normal as he was watching YouTube on his phone but then when I was trying to go back to sleep I noticed his phone kept vibrating and the video kept stopping as he was replying to the messages so I asked who it was because I was annoyed someone was texting him at 4 in the morning then I found out it was a girl he used to be friends with around 2 years ago was and I questioned him after he gave me his answer saying that she went to add him on Facebook and noticed he was in a relationship with me she texted him . So as I’m up for work at 6 I went back to bed upset and even more anxious then when I woke up he told me how much he loved me that the only person he wants is me and then she texted him again while we were laying down and he then only showed me the message Which was if I get what I want someone else gets hurt and then she said goodbye and he scrolled through the messages of last night quickly and read a few but I couldn’t help but notice a few texts which stood out to me and which he tried to reword to me saying before he got with me he reached out to her but stopped because he was unhappy in that relationship but the truth is the message says that I’m unhappy in my relationship that’s why im texting you I also seen that he rang her too yesterday so I know I shouldn’t have do this but when I left the room and left the phone I read his messages to her and it said that on the phone yesterday they said they wanted to be together so don’t know what to do

I don’t know how to confront him because I thought we sorted this before I left for work but then I seen all that and I don’t want him knowing I went through his phone

Please help me please