FIANCÉ CAUGHT Sending 'Our' Flowers to His OTHER Fiancée!

So I found out Mark’s been sending flowers to someone else. Not just any flowers, the *exact same* kind he always gets for me. We’ve been together five years, engaged for six months. Planning our wedding for next spring. Everything felt… good. Normal. He’s always been so thoughtful, always remembered our anniversary, my birthday. Never really gave me a reason to doubt him. Until now. Last night, my laptop died, and I needed to order some work supplies. I asked to use his. He hesitated for a second, just a tiny pause, but then said sure. His browser was open and, I swear to god, the first tab I saw was for "Bloom & Grow Floral Design." My heart just dropped. I clicked on it, and there it was. An order confirmation. For an arrangement of white lilies and purple hyacinths. My favorites. Ordered for a "Sarah Jenkins" at an address in the next town over. And the card message? "Thinking of you, can't wait to see you soon. Love, M." My fucking stomach just twisted. I scrolled down. Multiple orders. Monthly. For the past, like, eight months. Always the same flowers. Always to Sarah. All from Mark's card. I just closed the laptop. Didn't even order my work stuff. I'm sitting here on the couch, pretending to watch TV, but I can't even see it. He’s in the other room, playing video games, laughing with his friends. Like nothing’s wrong. Like he hasn't been sending *our* flowers to some other bitch for nearly a year. I haven’t said anything. My hands are still shaking. I don't even know what to do. How do I even start this conversation? How do I even *look* at him right now?