A couple of days ago my bf's phone went off late at night, I saw the message thread and I considered it to be inappropriate. I immediately woke him up to confront him. I've had previous trust issues with guys and this really shattered me as I did think I could trust him. There has been some background with this girl, where she messaged me last year saying because of me she couldn't speak to him anymore.

My response at the time being 'he can speak to whoever he likes, who do you think you are to him for me to stop your friendship? He had informed me he was texting her and that she had a habit of being flirty, but the message thread I saw showed him doing all of the flirting and her not encouraging anything. His 'excuse' is that he felt he was putting more into the relationship emotionally than me and now he knows this isn't true he'll do anything to win my trust back.

We have been together about a year and as far as I know this hasn't happened before, but I'm struggling to deal with it and don't know where to start with trusting him again. Any advice?

I doubt any one would completely trust some one who flipped the blame for their bad behavior back to the partner. That's unfair, and dishonest, plus you already have experience with trust issues.

You better find a way to talk more with this fellow so at least he knows about boundaries, and honest communications. How else can you both resolve this situation except bt talking to each other?