Cold Feet or Real?

Disclaimer: this is absolutely crazy, and I'm fully prepared for the heat about to come my way. I'll bare that if it means actually getting some compassionate advice. Also, it's a long story. Question first: Did you ever feel like "the one" was someone else, and still marry your husband/wife? Or did you call off a wedding because you realized this?
My situation:
I've worked in my company for almost 6 years. When I first started, I met a man (let's call him Matt). He'd been there a couple of years, and worked in the same area but not directly with me. We clicked instantly. I didn't want to date anyone, but couldn't stop myself from realizing how perfect for each other we seemed. We got along great, and I always had a better day when I got to interact with him. Awhile later I found out he was married, and I was so disappointed. He didn't wear his ring (his job was manual, so no jewelry allowed) and never brought it up, which was strange. I found out from others that it was mostly a marriage of convenience. She was a single mom, they dated for years with him treating her kids like his own, never had any together, and then had somewhat recently gotten married (she proposed). I eased off flirting, but we still became really good friends and talked every chance we got about everything under the sun. I ended up meeting my fiance a couple of years later and had a wonderful relationship from the start. Best friends, lots in common, no red flags, loved each other's families, almost the whole package. I always had a little tickle in the back of my brain about the things we didn't click on (I love the beach, he hates it. I love working outside, he can't use a hammer. I like adventure, he wants to stay home), but I felt like everything that truly mattered was perfect, and I wanted to be with him forever. I pushed Matt ever being more than a friend to the back of my mind and had a great 3 year relationship. We got engaged & are almost finished planning our wedding that's in a few weeks.

A few weeks ago, Matt changed branches. I was devastated. I'm still devastated. I realized that we were way closer than I was letting myself see, and so did he. We started talking via text, and it became nearly constant. The flirting intensified, and we eventually confessed that we'd always wanted each other. I was a wreck, and so was he. I felt terrible, and simultaneously like a huge weight had been lifted. I'm not exaggerating when I say that I feel like he's the WHOLE package. Everything that the core of me wants, he's it. All the "important stuff" but also the fun stuff. The things I love that make me feel alive, he loves them too. So now the dam has broken. Everything we've kept bottled up for nearly 6 is boiling over and I have no idea what to do. I never want to hurt my fiance, and yet here I am having an emotional affair weeks before our wedding. I'm everything I said I'd never be.

I'm filled with doubt. How was I so sure 2 months ago, and now I feel like I'm making a huge mistake. I can't call this wedding off. For one, it's all already planned and paid for. It's a pretty big event for people like us (a $10k + wedding is way bigger than we planned, but here we). Second, what if I'm wrong? Maybe this is fleeting and it will go away. Maybe I'll forget about Matt. He's still married, so it's not like he can just walk away. But about me, if I'm having these doubts then I can't be a good wife. He deserves better than what I'm giving him. He wants monogamy, and I'm afraid now that I'm just not that person. Maybe I'm just not a forever person, which I've always thought anyway.

Maybe I just needed to get it out there since I can't talk to anyway else, but if you made it this far... What in the actual F#@K is happening? Hit me with your real thoughts. It can't get much worse on my end.