How to stay happy in a Long Relationship (my list):
1. Let go of the idea of a relationship needing to last for a given amount of time to be valuable.
2. Enjoy aspects of the relationship that are authentic and come without strain, let everything else go.
3. Own your own shit, and stick to boundaries about not taking on other peoples shit.
Thanks for addressing the original topic, Marcus! I was quite interested to hear people's ideas about dynamics that arise in relationships that go on for a long time--less so in considering questions of selflessness.
Your point about relationships being valuable no matter their duration is very well taken! It's something I really believe in, but often forget to bring up.