Learning to trust

What are ways that you ladies learned to trust the man you’re with after they broke your trust? I know this is going to sound dumb, but he was caught texting a girl back when we were in high school (5 years ago) and ever since then it’s been hard to trust. It may sound silly, but that affected me when I was 15 and I still to this day think he still thinks about her, or is doing the same with someone else. He’s proven himself to be faithful since then but I keep having days where I stress and think, “what if” I tried doing the whole, not looking at his phone thing but my brain gets to me every single time and tells me that there is something and I literally have to check his phone. If not that, I check his location every 45 mins just because I’m scared. Every second I have to ask him for reassurance or else I think I’m losing him. It’s sooo unhealthy but I love him so much. He tries to work with me but he even admits it’s a bit over bearing for him asking so many questions and he told me last night if I don’t trust him that much I should leave.. please help guys :/