Advice please

So I’ve been with my partner for almost a year. At the beginning of our relationship a guy messaged me something quite flirty now I blocked him straight away and my bf has been struggling to trust me since even though o didn’t reply or respond to that guy, a few months ago he left his phone at my house for over a week he didn’t care that he didn’t have it, I didn’t look through it cause I trusted him but yesterday he left his phone at mine and he was hurrying to get it back so I looked at it and seen that a few months ago he was practically begging a girl to go to her house, now I love this man and up until yesterday I had zero reason to doubt him or not trust him how do I confront him about this I know looking at his phone was wrong and I do feel guilty about that but at the same time he has been messaging another girl and i cant just not say anything any advice would be welcomed