How to deal with the boss you can't trust anymore,

XDJM,here is my story. My boss tried to promote me last year, but didn't make it. Early this year she tried again, again it didn't work out. Now, one coworker will be leaving in two weeks and the boss will add the part of the work we shared to me. And again, she said she is working on to make this position higher level. How can I trust her again? What should I DO with this kind boss. I'm thinking to talk to boss' boss. What do XDJM think about this?