How do I trust him and relax?

Me and my boyfriend been together for a year and in the beginning he lied to me a few times and stuff with his ex happened and so on. But ever since that, he’s been very much trying to prove to me that he’s always telling the truth.. He’ll say something and I automatically think the opposite and been causing so many fights and causing him to give me constant reassurance and i’m surprised he hasn’t left me for how much I doubt him and think he’s lying. He isn’t the first guy to ever lie to me, so it’s hard. I got over all my ex’s did, but getting over what he did is what I need help with. He sees me everyday, we have each others passwords to social media... I literally stress to the point that I overthink everything. I don’t want to end things because it would hurt a lot, I just need advice and help because its eating at me. How do I trust someone and stop freaking out so much and overthinking and needing constant assurance when they’ve lied to you a few times...