Your overall situation sounds like mine (or most others if remove the details, and get to the root of it)... and yeah, once you are woke to how marriages / relationships play out, you realize how naive / innocent you once were.
Don't beat yourself up too much. I've come to realize that even if I was 99% perfect, I might still be here.
Some people just get bored over time, focus on the negatives, and fantasize about other possibilities - taking what they have for granted. They are not aware of their own issues, and lack self control and boundaries. Once they fall into the trap, they are just victim to the cycle of brain chemistry / hormones.
Can you learn from you mistakes and be better? Yes.
Will you being better / perfect guarantee that it won't happen again? I'm inclined to say No, unless both people are very well educated in the dynamics of a human brain and MR, and able to self identify / regulate their feelings.
