I understand you example, and I try to approach things like that, but its more primal than that, just the mere unknown of it put it out of my comfort zone, the simple fact I have no control or hand in it is the threat.
I really do understand. Cognitive therapy gets very hard when you hit the 'real' stuff - exercises that were relatively straightforward become incredibly difficult. You aren't at all alone in this. The best encouragement I can offer is that this is actually how it works. You just keep taking tiny, tiny bites out of the bigger problems. The more frequently you practice it, the better your mind gets at it, and you will start to challenge your negative thinking automatically. It takes time - there's a reason why DBT programs operate in terms of months, not weeks.
but this assignment won't change that, it's going to take more than the suggestion of a different way of approaching something (willingness vs willfullness), its going to take creating new memories to replace the bad ones, changing my core values and beliefs by challenging them and addressing the source of there creation.
This assignment won't change everything in one fell swoop - that's true. But this assignment is a part of changing those big things, yes.
Memories aren't replaced so much as they are diffused. So, adding new positive memories/associations 'waters down' the old, traumatic memories until they simply become part of your past, and not your present. Every small positive cognitive action contributes to the watering down of negative core beliefs.
And no-one can be perfect at it all the time. I'm writing this for myself, just as much as I am for you. Sometimes I really want to stop moving all this salt in my wounds one grain at a time - that's what it can feel like. But it does make a real difference.
It's sort of odd to say, but the more uncomfortable the work is, the closer you are to the important stuff - I believe this, anyway, when it comes to trauma work in all it's forms. Only you can know when your distress tolerance is shot and you need a break. But take those breaks by distracting and soothing, not by allowing your mind to run away with you on the worst-case scenarios.
You're doing really great work. Rest when you need - and don't give up.