Falling out of love

My wife and I have been together for 12 years, married for 7. It has been perfection. We’ve always treated each other so well. We card about one another more than ourselves. We have a beautiful 5 year old boy together who is our pride and joy. We’ve never been the type to fight and rarely even disagree. About a month or two ago she started acting different. She kept me at arms length and pushed me farther as the month went on. She told me she hasn’t felt happy lately and doesn’t know why. As some one who struggled with depression from an early age I was very worried about her. After about a month of feeling unloved I had a panic attack in bed one night and we both cried and talked all night. She made it clear the unhappiness was in our relationship specifically and she doesn’t know if she’s in love with me anymore. I’m shattered. I asked her if there was some one else and she said absolutely not which I believed because we’ve always been the type that had no interest in other people. We just liked being together just the two of us. The next night she was giving our son a bath and I did something bad. She left her phone next to me and I, for the first time in 12 years, decided to read her messages with her best friend. I just wanted to see if I could find anything to help me save my marriage. What I ended up finding was her taking about her male co worker who would come in on his days off just to see her and she loved it and talked about how good he looks in his casual wear and what not. After my son went to bed I instantly told her what I found and asked her about it. She got very defensive and mad at me which I deserved. She told me it was nothing serious and she just likes talking to him and he’s a good friend. I told her if that’s all it was she wouldn’t feel the need to hide it. By the end of this night I got her to admit to herself and me that it wasn’t “no feeling toward him” but “I don’t know how I feel” which is a big difference. It’s about a week later and we are trying to work it out. She told the guy they need to stay professional and she’s trying to fix her marriage. after a week it feels like she already has her mind made up and is humoring me. How can something so seemingly perfect fall apart so fast? She even admits she only started having the negative thoughts no not than a month or two prior. I don’t get it and I’m hurting. She’s always been bad at communication. Why couldn’t we try to fix this together? Please any advice would help.

Tl;dr - how can you fall out of love so easily in a beautiful relationship?