First relationship: Lasted about six months; I was too young (18) and desperate to start my own life. I learned a lot and achieved independence, but the relationship was in every sense of the word a failure.
Second relationship: lasted almost a year; I was still too young (19) and, because we never cohabited have considered downgrading this the way Pluto is no longer a planet. It ended unilaterally on his part, left me broken hearted: can't say I learned much.
Third relationship: lasted almost four years; even though I was 20, I consider this my first "adult" relationship. We shared all income and expenses, often to his detriment. The passion cooled quickly and we would up opening up the relationship after about 18 months: or love rapidly became fraternal rather than sexual. It was a messy break-up though we quickly regained a deep friendship. I learned a lot, and despite the nasty break-up consider it to have been a net-positive.
Fourth relationship was an affair that blossomed while still involved in relationship #3, eventually lasting seven years on and off. As he was completely incapable of domestication, we never cohabited even once we both were free of other entanglements. For years we'd sneak in and out of each other's lives until he disappeared; I considered him the one that got away. We met up toward the end of his too-short life but by then the passion was gone. Much as I loved him, I'd say that one was a failure.
Fifth relationship: lasted just over four years. He was the only son of a pair of wealthy Venezuelans. He taught me international travel and a broader outlook. Despite the fact that we quarreled constantly, I learned a tremendous amount, and when it was (finally!) over, we became best friends. I learned a lot from him.
Sixth relationship moved me to Paris to live with him. I learned acres and boatloads and he remains the high point of my existence. Had he not died, I'd still be living a highly combative and (probably) toxic relationship with him, as I loved him immoderately. Burying him remains the low point in my existence.
Seventh relationship was "coming home", as we grew up 18 months apart and within a half-mile of each other. It lasted nine years, though it should have ended at five. The net result was so toxic, I was lucky to escape alive.
The last two were brief and completely toxic. They are why I choose to live alone.