How do I get over major trust issues in my relationship?

We are together for 7 months and at the beginning he used to keep in touch with random girls he knows and a few times he hid the phone when one of his exes texted him. Eventually it all stopped after many fights but I can't relax. He was single for so long and only causally dating so he kept some habits that are dangerous while in a relationship.

I am always so afraid something will happen again that will disturb my peace. Also whenever we are separated physically I am obsessing whether he is still texting, hiding something etc.

I can't escape my own head. How should I deal with it?
How do I get over major trust issues in my relationship?
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