As someone who does psychic marriage and relationship stuff all day long, I couldnt help but respond because this query jumped out at me. You've only shared minimal details here, but there's a lot I can see absent of written information.
In any romantic relationship, there's goimg to be some ups and downs. There's always goimg to be at least a few disagreements in any relationship but it's the way the couple handles those issues that reveals comparability. To be honest, if a couple doesn't experience at least one little issue within the first few months of a relationship, I'd question the passion… but you're talking about 12 years — more than a decade — of knowing someone and experiencing some kind of back and forth pattern in the relationship.
There's this old saying about long engagement being bad luck and that rings true here. You've known this person at least 12 years yet it's not become permanent, permanently. In any relationship, after about 12 months (one year, not 12 years) I'm not so sure there's all that much “new” to get to know about a person. After knowing someone for about a years time, chances are, you know them pretty darn well. And after 12 years, I can pretty much guarantee to you that there's not much new to learn or discover about a person.
I realize within that 12 years you've made investments in time, energy, emotion, effort and even money. I also realize that because there's not all that much new to learn or discover about the person, you know what to expect in a relationship with them. For example, married men who have affairs rarely leave their wives for their mistresses. It's not that the wife is all that much better, it's that with the wife he's made a pretty big investment on a lot of levels and he knows what to expect in the wife and in the relationship. You know this guy pretty well and know what to expect. Not all times were bad, so the possible opportunity of reconnecting the relationship doesn't sound all that bad to you.
But let's look at the facts here: you've been off and on for MORE THAN A DECADE. Regardless of whether you've spent more time on than off or vice versa, the relationship hasn't WORKED YET — and, you've known them more than a decade.
If this relationship were meant to be so to speak, it would have been and would gave been permanent years ago, regardless of what obstacles were present. There comes a time when it's more prudent to cut one's losses. Learn those soul lessons the relationship gave you and move forward.
If you really feel that you've changed and he's changed enough to make the relationship work out this time, by all means give it a try. But there's another old saying that rings true here about a groove in a record. Chances are, that needle will go right back into the record the same old way. And in 8 years you don't want to be back here again asking this same question, sans the 12 year and calling it 20.
I hope this helps!