Here is how I see LTR in the world of TRP (posted as a comment in another thread, but requested as its own individual post):

In my experience, a woman will always remain "in love" (faithful, sexual, respectful, submissive, and happy) as long as she perceives your value (frequently referred to as SMV) to be higher than hers. This is the way she "keeps" you - by treating you the way you want to be treated.

There are two factors that can ruin the feelings of "love" that she has for you. One is higher value from another competing male, and the other is reduction of your value / increasing of her value to the point that you're no longer more valuable than she is.

So long as you're constantly striving to be a better man, you should never have to worry about an imbalance of value with her. The only other piece of the puzzle is the competition, which sometimes wins and sometimes loses. That's the nature of the game.

EVERY long-term relationship adheres to these guidelines. So long as you know the rules, playing the game shouldn't be terribly difficult.