The gap between expectations and reality
A simple way to think about trust in your personal or professional life:
Lack of trust comes from the distance between your subjective expectations and the universal objective reality. That could mean that you don't expect someone to follow through on a promise when they will (Type I), or that you do expect someone to follow through on a promise when they wont (Type II).
You can't expect reality to conform to your expectations. Reality is hard, and you can influence it but not control it.
The longer that your expectations fail to line up with reality, the less you trust the world around you. Maybe you'll even give up expecting anything at all.
You also can't expect to line up your expectations perfectly with reality. How could you? That would require knowing everything, or at least close enough to it.
So my suggestion is to broaden your expectations and loosen up what you anticipate from other people. Leave more optionality floating around. This is different from having no expectations. You expect some stuff, and you wait for reality to fit in those expectations or not. Now you're training your expectations.
Over time, you will train your expectations on real data - the reality coming in from around you. And your expectations get better and better and approximating and predicting reality. They'll never be perfect, but they'll be close.
As you train your expectations to reality, the distance between the two will shrink, and your trust will grow. And you will feel more comfortable and more prepared in new situations.