Why does my boyfriend always think im going to cheat?

I have been with my boyfriend for 10 months, in the beginning of our relationship we had the worst arguments because i had a friend who was a guy who also goes to the same gym as me, my boyfriend went as far as to read my text messages with this guy (which were completely normal and friendly, I knew this guy before I knew my boyfriend and nothing ever happened between us like that, a few jokes but mainly sarcasm) but after that he would always have a problem with me going to the gym and he asked me to block this guy on my phone, which I did, and then he would always check that I still had the guy blocked. Now that lockdown has ended and the gyms are open again im desperate to go back for a session but he is already saying things to hint that he wouldn't be happy if I did go to that gym, but that's where I pay to go and I like it. Even though I always reassure him (and I am with him after all) he always finds a way to make me feel like im doing something wrong or having something to do with this guy when I'm not at all?
Why does my boyfriend always think im going to cheat?
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