How do I stop being insecure and overthinking?

Anonymous
I'm a girl and sometimes I'm assuming my boyfriend is cheating on me. But he always proved me that he never do that, he's really open to me and honest where he wants to go. When we meet he didn't mind if I'm checking his phone (and if he text someone, he will lend his phone to me because he told me to read the chat) and if something is kinda sketchy for me he will give me an explanation. I don't know what is wrong with me, I'm always paranoid. I did trust him but sometimes the voices on my mind is too loud, it's so annoying.
How do I stop being insecure and overthinking?
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