You said that you have an inability to trust.

If people have let you down, you should not trust them - it is the appropriate response. You are not supposed to have the "ability" to trust people who have let you down.

If you are talking about not expecting that a fairly new person will let you down, you should simply consider giving the person the benefit of the doubt. "trust" is a wrong word. Then, based on the record of interactions with a new person you will or will not develop trust. So trust is a result of interactions. Thus talking about "inability" seems misplaced, as well.