Why doesn't he trust me? How should I oppose to his behavior? Did I made mistake slapping him?

I have been dating this guy for almost a year, Initially things were alright now things are taking ugly turns since I have another friend who proposed me, I refused him saying I am already dating someone. I conveyed all this to my boyfriend he told me I should block him, I was like when my friend is not harming me or forcing me why should I block him?

The guy friend agreed to me about moving on, and just on a casual note once asked me out for a lunch, I agreed and didn't tell my boyfriend because I didn't wanted any fights with him, I was aware what's the limit I have with my friend, so I just went for an hour.

Few days back my boyfriend checked on my whatsApp, he was literally reading all of my chat with him, he got to know that I went out with him, I clearified him everything that see only this this happened, it's was a casual meetup.

He was literally dragging the same topic for days, I felt like is he doubting on my character or what? When I clearly said him that nothing is there with that guy, I got too frustrated once that he is constantly judging me, I slapped him, and he slapped me back!

I am too much guilty about slapping him, but how should I deal with him doubting me when I did nothing wrong?
Why doesn't he trust me? How should I oppose to his behavior? Did I made mistake slapping him?
Post Opinion