Why do I have trust issues?

My Boyfriend and I are together since 4 1/2 years now. I know that he loves me. But sometimes I check his phone, his insta, FB... to know, that he doesn't write anyone else. Especially women.
3 years ago he was still writing his ex and send intimate pics to her.. I left him. But one year later he decided to re-established contact.
We made a promise, to talk openly and honestly about our feelings. So we still do.

We don't live together, so I have no controle over, what he's doing when we're separate for a few days. I always get a little jealous, even if he talks about woman..

So why am I that crazy to find somthing, that would hurt me?
Do I just worry too much about the past?
Why do I have trust issues??
Why do I have trust issues?
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