Let's start by thinking about what trust is. You may disagree with me or be able to take this thought further but the best I could come up with when I really went into the feeling is:
Trust is what I hope for in all relationships.
It means that I will never be hurt by anyone.
It means that people will always mean what they say and do what they say.
It means that they will always know what I need and deliver it.
I'm sincerely glad that I don't have to ask for what I've defined above as it felt completely unrealistic even as I wrote it. Because...
We are all human.
All humans make mistakes.
The greater percentage of human beings mean well.
Most humans mean what they say until they realise that they can't do it.
Most humans mean what they say until they realise that it had a different meaning for you.
Most humans try to be nice by saying yes but sometimes they really meant no.
Relationships have ups and downs.
A lifetime is a long time to love one person.
Hopes and dreams change.
Forever is a long time to be perfect.
Nobody is in your life to serve your purpose, not even your children.
Stress affects all relationships.
That children comment is a tough one, but at the end of the day everyone is an individual even the child you gave birth to/sired, and nobody is born to make your life ok, to be perfect around you or for you. No one, not even you. You can't say the words "I'm only human!" without remembering that so is everyone else.
Even you don't get things right for yourself all the time. You make wrong decisions. You say you'll do something and you don't really want to so you find a way out that fools no one. You have changed since you were born, maybe more than once, and you will do so again. If you're honest do you love him/her the same way you did when you met? Were you really surprised when it broke up or deep-down did you know?
Right, it's a start. If this makes any sense to you or you can rethink it to make sense you will immediately see that we have high expectations of ourselves and other humans.
Perhaps the best start with dealing with trust issues is to realise that no one is perfect isn't a cliche it's the truth.