I always find it strange that a woman's heart could wander, but I guess that is being sexist!
I do agree you need to figure out what you were not doing, that hurt her, and what you were doing that hurt her. Sins of commission and omission. Were you loving her as Christ loves the church? Were you treating her as a special, one of a kind, and irreplaceable woman?
Are you separated already? If not, surprise her by taking her out, or buying her flowers, or whatever she likes. That is not bribery, it is appreciation. Write her a letter telling her what she means to you, and send it to her. Make sure it is on pretty paper! LOL
But more than buying or doing something, are you loving her every day? Do you let her know when you wake up, when you come home from work, and when you go to bed, how much you love and appreciate her? It doesn't even have to be words, even presence can mean a lot.
Then there is her SAD. (Seasonal Affective Disorder) Is she getting medical help for it? I grew up in Edmonton, Alberta, and I always went through SAD in the winter. I moved to Vancouver BC, and never had it again. Moved back to Edmonton, the second year there, I realized I had SAD again. I took a lot more Vitamin D (many people are very deficient in this vitamin!) And eventually I had to take 1/4 of an antidepressant for late Jan, Feb. and March. I moved back to BC, it is much warmer, so I didn't get it here. So, she really needs to see a doctor and get on an antidepressant!
Lack of sunlight in the winter is a very real issue. Lack of sunlight also means lack of Vitamin D, which causes all kinds of physical and mental problems. If you have the money, take a holiday to somewhere warm with her, like Florida or Mexico. And do a lot of walking and sitting by the pool! IN THE SUN!
I do hope you can win her back and restore your marriage. But do NOT get her back and then go back to living the life you were living. If you want her back, you must make a life commitment to treat her like Eph. 5, as Lynn posted above!