For me, trust takes time and figuring out how the particular t responds to things. I know there's always a finite nature to the t relationship, but I always hope it will end amicably and when I am ready. I do know however, that things come up in life and t might leave our I might leave before we are "done". With last t, I realized I trusted her when I was able to look to her for support, and know that whatever form that short came in, it would be in my best interest. That's not to say it was ready to see in the moment and I didn't struggle, but I kept going back, so I'm guessing I trusted that she would know what to do or figure it out in short order... in the end, I realized I had trusted her when she told me something I had never heard from anyone else, but desperately needed to hear (and I didn't have to ask her to say it, nor had I ever brought up really needing to hear it)...
I hope you can find that trust in your t.