Twin flames are not necessarily life partners at all. They are there to teach you a lesson and make you obtain your truest of selves. That's the difference. Life partner relationships are when everything is aligned and healthy. But, twin flame relationships awakens but can also be very toxic. So, it's perception really. Life partners come to you when you are ready. In relationship, the best way is to cut through the illusions and see what is really there as far as being able to build a long term relationship. It's the "little things" and not the "big bang" that really makes relationships lasting. The distinct feature is sense of safety. There's more safety with life partner than a Twin Flame because you take time to build that trust. But, with twin flames, you don't necessarily build that trust and the earthquake that shakes up your life can break the relationship very easily.
So that is the difference. It's better to take relationships as they come and appreciate the connection whatever that may be. Gratitude is the word here.