Quoted:
Intimacy. Have that and the rest will come easy. Been told that by a few women again and again.
This.
Emotional intimacy is created through a series of deliberate actions on your part, and very easily destroyed by actions or lack thereof.
The issues you described are typical marriage issues which can usually be solved through counseling and other means. There is always more to the story than you were "just a nice guy" and "did everything she wanted." Truthfully, most women are unable to say what they want because they can't reduce it down to their core emotional need for emotional intimacy.
If there was an issue of trust being broken time and again, however, things can change. Perhaps your attention and priorities were different than you think - putting too much emphasis on work, looking at other women, not pulling your weight in ways
that spoke to her.
A few good resources that can be helpful:
The Five Love Languages
Laugh Your Way to a Better Marriage
Myers-Briggs Personality Assessment (an accurate knock-off can be taken for free at mypersonality.info )
A GOOD marriage counselor
Love & Respect
Through those resources, I'm sure you can narrow down what the root cause of the issue is. Perhaps there was infidelity on one or both of your parts, emotional or physical. Perhaps she is having a personal growth period and she feels a loss of purpose or direction.
Regardless of whether you stay married, figure out what the root cause is now so you are able to heal for YOU so you can be the best you possible. She may or may not come around, and you may or may not want to save your marriage. Either way, you're stuck with yourself at the end of all this and getting some support is always a good idea.