Why cant I trust him!?

I love my man to pieces and have been with him for nearly 3 years, but I just dont trust him. I feel like if a girl were to hit on him he'd go all in. When we first started dating he used to hide his phone from me, my friend even found his tinder account which he told me was just an old one😒 but to this day I just feel like he'd cheat on me. Everytime I go on his phone even if it's just to look something up he grabs it off me and says "oh I'll do it" or just watches me like hes nervous. I'm very insecure and he knows that but when we were arguing he still had the guts to say "I look at girls in magazines and am more attracted to them" I just dont know what to do anymore. I love him to pieces but how to I overcome this?

Side note: I'm bringing this up because I asked him if he could feed my friends dogs (they pay us to) and hes been there for nearly an hour talking to one of the housemates who's a girl. He wont answer my calls and messaged me that hes busy talking to her and he'll get back to me later.