How do I stop feeling this way?

I have been happily married for 10 years to a man I love deeply. We have had ups and downs of course, but nothing that hasn't made us stronger. About 3 years ago I joined a gym, and became friendly with one of the trainers there, who is also married. After some time I started training with him, and of course we got closer as a result. From the very beginning our relationship has been a flirty, somewhat inappropriate one. But nothing has ever happened or really even been discussed. I know he finds me attractive, and he knows how I feel. We both acknowledge the temptation we face, but other than that, it's the giant elephant in the room every time we see each other! As time has gone on, I have found myself having deeper and stronger feelings for him. I miss him when we are apart, I think of him and care for him a lot. We have lots of fun together, and have a lot in common. I don't want to create problems for my marriage or his, but most of all I don't want to lose him as a friend. He seems to have been able to put his feelings of attraction (and lust/desire, whatever it is) aside, to some extent, so we can maintain our trainer/client relationship and also our friendship. Although he flirts like crazy and makes no secret of how he feels, he's obviously able to draw a line and go no further. How?? And how do I do the same? How do I move past these intense emotions and desires?