I have to echo the comment, What is your definition of successful?
I think it all has to do with how much one person can take. Some people lack boundaries and can tolerate abuse much longer than others. To me, that doesn't equate as successful, just "plugging along". I know BPD's in relationships for 30-40yrs plus. These people stayed together because it was how that generation was raised, or they were practicing Catholics (I am not knocking Catholics, I am one

The non-BPD grew to resent the BPD and the marriage was terrible, yet they stayed together into their 80's, until the BPD passed of Parkinsons.
40yrs in a terrible marriage that produced five great kids. Yet, the non became an alcoholic to cope and was never happy... .for 40yrs.
In my own experience, my ex has stayed in relationships longer than ours (ours was 4yrs with over 20 breakups). She's even lived with people a few years... .
but after knowing three of the exes we all suffered the same fate and she didn't treat any of us better than the next. All of us were pegged to our replacements as abusive rapists.
It really doesn't change for them. No matter how long the union, they are NEVER truly content nor happy.