Hiding a Secret from My New Husband

I recently got married to my high school sweetheart after being divorced for five years. We've always been close and supportive of each other. But there's something I haven't told him yet, something that happened before our marriage that makes me feel guilty and scared. During my divorce, while things were still complicated with my ex, I started seeing someone new, someone who was a huge relief at the time but now feels like a dark secret. We never crossed any hard lines, but it's not as simple as an affair because we did roleplay some intense stuff that I'm worried he wouldn't understand or accept. He’s such a good man and I love him so much more than my ex ever made me feel. But how do I come clean about this now? Should I tell him everything, or keep it buried forever?