What i think is that maybe Skylar didn't really love Walt at all. They did not look to me like a couple that ever had this real feeling. They looked like reasonable people that were reasonably in love with each other. First of all, Walt seem to be a passionate person inside. It is obvious that his first and real love was Gretchen with whom he got close on the basis of a common interest which is chemistry. I assume that for Walt, being a creative and passionate mind, to fall in love with a woman would need her to appreciate and share his ideas and to admire his geniuos. (Walt is a bit of a Dorian Grey which I guess is fine considering the greatness of his mind. Remember, how he wanted to look badass and how carefully he nurtured his Heisenberg style and how much pride he took in being the best meth cook.) Gretchen did so (probably, she was able to do so as she was also a science person). Skylar did not for she was a person who has nothing to do with chemistry or science, she is an accountant from the world of business which makes her closer to appreciating Ted's businessman talents (if there were any. You see, I strongly despise Ted). Also, she just never looked to me like someone who really understood the contribution Walt made to the field of chemistry or realized that Walter was a genious (or at least a major chemistry mind).
So, Walt and Skylar landed together as a husband and wife. And I guess Walter grew to really love Skylar (who was, at first, probably just his way out of post-real love period he had after being betrayed by Gretchen and what's-his-face guy with huge ears. Oh, got it, Schwartz guy). You see, he gave birth to his first and onliest son over whose cerebral pulsy they fought together for many years and so on, so they have been through a lot together and Walt appreciates things like this. I guess he appreciates loyalty. And trying to be loyal till the end. But Skylar, vice versa, over years, stopped feeling attractiion to Walt. Guess she got dissapointed in him, cuz he failed to be a man who is able to provide her and her family with a better life. So, she fell under impression of Ted whose outlook was so shiny and whose floors were so warm. Thus, she tollerated his illegal behaviour better than Walt's. She seemes to understand why Ted did what he did and she seemed to completely ignore the reasons for Walt's crimes. Though, Walt admitted in Felina that he did it for himself I think for at least a season or two he was really doing it for the family. Oh, and buy the way, Ted did his crimes for himself, only to maintain his lifystyle and probably to feel that he is a no-worse boss than his father was. It was bullshit about the personel and family he cares for.