There is nothing at all you can do to ensure that someone won’t leave you.
In fact, they will, when they die…unless you die first.
But ignoring that bit of unpleasant reality, there is nothing you can do to ensure they won’t leave you for someone else. Or for more than one somebody else. Or for nobody.
It’s not a matter of trust because nobody can say what the future holds. Nobody can say how they will feel when they meet a new person. Nobody can say how their life might suddenly change. Nobody can say how they will feel about things 5, 10, 15 years from now. I can only make my best estimate of that, and act accordingly.
I can only pay attention to my partners’ needs and meet them to my best ability– as long as doing so does not conflict with my own needs.
The only thing I can say is that I trust that my partner(s) will leave me if they need to, if I no longer meet their needs. There is nothing to blame on anyone if that happens. As long as I tried my best. Though I can’t stop anyone from leaving me and the door will always be open, I will always try to be mindful of ways I can make their lives better with me than without me.
I trust that my partner(s) will do what is best for them, and treat me with kindness in the process. If I can’t trust them with that much, there is really no relationship to speak of in the first place.
Trust is important. But it’s impossible to trust anyone not to leave.