Hi there redrook18, I'm really sorry to hear that you are experiencing so much distress. Before we head into the discussion, do you have any self-soothing or distress tolerance techniques that you know that you could use? I'll put some links here for you that might help if you need some/some more:
Also, it may help for you to have some distracting non-sexual media you can redirect yourself to so you can give yourself a break from engaging with something that is bringing you distress. If it's media with a narrative, a puzzle, particular genre of game etc. have some cued up, or other activity that engrosses you, have that at hand.
Moving into the reason behind the distress you are feeling, do you have a sense whether it is the imagery of the fantasy is making you feel distressed, or is it more the idea that you have a kink for this fantasy, and how that might be received by others and/or thoughts about what that might mean about you as a person? If you're comfortable sharing this, even if the answer is that you're not sure, this can help us take the conversation in the most useful way to you that we can.