it can be hard to talk to a T about stuff we havvn't totally come to terms with ourselves. It gets easier if you've been able to build up some trust wiht your T. Being a DBT T though, she may focus more on the behavioral aspect of things rather than any emotional validation. You could try talking to her about the anxiety around telling her your fantasies, and what you would like to get from her (especially if it's different from what you normally get). I've bene known to tell a T that I just want to talk about something, be heard about it, and get some validation without having to think about interventions or "solving a problem".
I'm sorry your relationship isn't going well... That can be a huge added stress on top of everything else. I know when my relationship is on the rocks or more stressful, everythign else is impacted.
Hope you can find a way to talk safely about what you need to.
Also, if it helps, can you write it down and either read it to her of give it to her to read? I know that helps me when I have trouble verbalizing things in person.