Is there a long marriage on record which hasn't survived some woes? I listened to people who once reported true love and tingled at each others' sight (or site) complain bitterly about their disappointment. The trouble is in expectations. Few people sustain all that passion, especially into their senior years. Troubles interfere, like health issues, differences of opinion and goals, low self-esteem, insecurity, etc.. So, the marriages which do well are flexible enough to expect less, try to maintain a connection and restore a little romance when possible, avoid external temptation and work together. Compromise rules.
I thought about old, good marriages. Those couples have a good balance between independent activities and togetherness with more of the latter than the former. They appreciate each others' strengths and downplay the weaknesses. They empathize with each others' hurts but not always evenly; maybe he seems to care less this month but did fine less month and vice versa. They have all weathered storms and some considered leaving, but they came through those difficult times with renewed commitment.
I thought about marriages which didn't make it. They weren't that different from the good marriages for a long time. However, they held grudges like little ulcers that festered and grew. They reinforced their hurt feelings when there was any little slip-up or when their own ego feel short and they used complaints against the other, often magnified, to feel justified in wanting a fresh start or a new admirer. They gave up and didn't stay.
The trouble is that whereever they went next, they took themselves along. So, alone or into the next relationship foray, no matter how great it started, unless they really did some great soul-searching or found a totally different type of match, they still found that life and relationships requires patience and work.