There are never any guarantees in relationships...
But I must admit, the less grounded nature of my secondary relationship does sometimes cause me certain irrational insecurities. My primary relationship isn't perfect and has had ups and down over the years but there is a certain comfort in it because it's the 'official' one. We share a home, bed, kids and other responsibilities. I think in my head, the secondary love can be more easily threatened by new (or even existing) partners because we don't share anything concrete (ie. we live apart and just 'date).
Since there is no way for either of us to join the other because we already have primary families, it feels like we're at the mercy of the primary partners schedules or various other family duties where a secondary partner just isn't included... Being somewhat in the closet to some relatives doesn't feel great either, but it's reality. And sometimes it sucks, but the love makes it worthwhile.
I guess the key is to fight off the bad thoughts and concentrate on the good.
Change is what happens when the pain of remaining the same becomes greater than the pain of changing. ~Marilyn Schiltz~