How do i get over it?

Hi IMs,
this is going to be a little long, so please bare with me.

I have what feels like it should be a minor issue; but for me its a lingering issue.

My bf (love him beyond words) and i have two kids, loving life and moving on and growing up together is amazing. He is for the most part my best friend. I can't fault him except this one thing... this one person.

He had a female friend. they've known each other since they were teens and she was his first. they grew up and were still friends on and off. they were in contact when him and i were, what i thought was pretty sure we wanted to be with each other. Admittedly, i snooped and found a message to her suggesting that he hoped to find someone like her - we'd been together 12 months (give or take). Later, in our relationship he was visiting his friends, quite a distance from me, and would deliberately leave his phone at home while he caught up with her.

it caused problems and yes, this is compounded by my experiences in previous relationships (cheating, lying and secrets)

Again, it came up and he was seeing her, as friends, honestly i think quite innocently, in public etc but still wasn't upfront with me. he tried to hide it. and was caught.

we've been together a total of 5 years... the keeping it from me has has happened on more than one occassion...

I have tried to talk to him and explain that i feel betrayed and less important than her. I feel like, for him, that my feelings of betrayal are less important than his need to be her friend.

My question sisters, is how do i get over it? How do i stop letting this insecurity or distrust or what ever this feeling of helplessness keep getting the better of me?
please also