I cruised through the first four pages of this thread, then skipped right to the end because, frankly, I don't have all day for it.
But I do have strong opinions regarding lovers and relationships of the stable and long-term kind.
First: I believe that it is a myth that there is only ever one person for the other. It's horribly unfair to both yourself and the other person to expect (and eventually demand) that one person fulfill all your expectations in life. By such a standard anyone would fall short.
Second: A person's needs and goals shift and change as s/he grows and learns. This is not a selfish thing necessarily, but a pragmatic and realistic, reasonable thing. The person best suited to you in a certain time in your life may not be the best person five years later.
Third: Enduring relationships are all about compromise, but it becomes very difficult to compromise continuously without eventually feeling compromised. I have always said that relationships are the balance of the needs of three separate and completely equal parties: You, the other person, and the relationship itself. Knowing when it's in the best interest of the relationship to compromise (or stand firm) can get really sticky.
Four: Bitterness is the greatest enemy of finding the right person for you at the right time. Learning from a past mistake without making sweeping generalizations about a specific sex, race, body type, etc is one of the most difficult but necessary lessons one can try and accomplish.
I can give two examples of enduring, permanent relationships. One I know well, the other I have observed over many years as a complete outsider:
1) Paul Newman & Joanne Woodward. By all accounts they have a happy, stable partnership that has grown more intimate over the years. It would be tempting to say that each was the destiny of the other, that they were made to be together. But what if Paul had been a mechanic instead of an actor? Even if their lives had crossed at the same point in time, would they have been able to make it work? Would they have even considered each other as a possible mate?
2) My roommate and his best friend/business partner. They met in college over twelve years ago and have survived changes that would swamp any normal partnership. She stood by him when he married a woman (and the divorce that inevitably followed). She was there when he came out gay, took care of him when he got dreadfully sick and almost died, has weathered his multifarious relationships with men who ranged from difficult to impossible. They run a very successful business together and spend most of their downtime together, besides. Aside from sex (which was never in the picture), they are very much a power-couple and cannot imagine life without the other.