IMO, when we spend a lot of time together with our partner, we tend to take each other for granted, start finding things we don’t want to tolerate or find annoying about our partner and even think that the ‘grass is greener’ with someone else. When we are with someone and don’t require much of their attention those things usually do not exist. We never waste time on such things. Intimacy and romantic love increases but the problem is commitment for each other may not be there for long. It feels new and exciting every time we meet. I had a little of that in my other relationship before we moved in together. After living together, intimacy and romantic love was non-existing.
Your situation was not like that (living together) because it was a LDR but most people are not able to deal with distance very well. Unfortunately, I feel it is much more than meets the eye in your situation. There is much more to mine as well, I can guess. Your story does sound very familiar sometimes.