Ohhh this sounds like it causes a lot of emotional trouble on you :(( sorry to hear that.
From what you say, you seem to care a lot about the person you're with, so maybe you could try to explain to them how you feel and that this doesn't necessarily mean that you won't want to do things ever, just that you don't enjoy the pressure of having to do it, without a choice since you're in a "romantic relationship". You don't even have to have a clear idea of where in the spectrum you are to explain to them how you feel. I think you just need to be clear about it, about the fact that you get disenchanted and that this has happened before (to make it clear that it isnt because of them, your brain just works like this).
Ask yourself if you consider breaking up because you dont like what you have, or if it's bc you want to relieve yourself from the pressure of having to act romantic with them, and then decide if breaking up would be a good decision for you or not.
If the person you're with hears you, and understands your side then im sure you can reach some sort of middle ground, setting down some limits as to where your disposition goes and findng out which things you're more comfortable with than others.
I dont think breakup needs to be the Plan A solution, so i hope talking about it helps!