What can you do to repair broken trust in a relationship?

My boyfriend got caught hiding some really important aspect of his life from me even after 3 years of dating. While the lie was not fatal, it’s making me feel like I don’t really know who I’ve been dating all these time because it was a huge part of his life that he hid from me. He really wants to make it up to me and have been super open/honest but I’m having a hard time fully trusting him again. Any tips for both him and I to repair the broken trust?

What can you do to repair broken trust in a relationship?
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