To be trustworthy means

1. I will do what I promise I will do and if I am not able to do it, you will be the first one to know.
2. I will not over promise and under deliver and if it is going that way, you will be the first one to know.

ps:
What if I Was not able to do what I promise to do but told you about it in advance?
In doing so, you have earned my trust but hopefully not being able to do it would not become a habit.