if she is an INFJ, then it's likely that she actually fears the disappointment of having the trust broken. In that moment of discovering that trust has been broken, the pain and hurt is pretty unbareable. I think this is why she "blocks" it rather than "sabotages" it.
It is to protect herself. If you don't understand this, that's ok, I'm just thinking as an INFJ, and I too can see myself in her. Also, like myself, it's possible she has experienced this lots of times, so she just has made this pact with herself to not ever be in that position again, as her empathy helped her also see how her expectations of others caused her her own pain... I again can relate. So, it's not being imature or inadvertantly being manipulative, more like setting a block or a boundary of a guard of some sort, to avoid the inevitable.
It is not disappointing when enimies break your trust, it is also not something that will bring with it, the anticipation of knowing you'll (she'll) get hurt by someone she thinks well of, or wants to think well of. I bet she would love to be able to trust, but she knows how she feels and that emotionally, she's just not safe to do so.