Hi KG, I also got the same feelings and totally understand where you are coming from.
For me I wouldn't let myself reach orgasm as I feared the feelings you describe so just enjoyed pleasing my partner. When in private and had an orgasm (dressed) it was still similar feelings but they did subsude after 30 mins to an hour or so, so I am of the opinion of the endorphine argument.
Since being on HRT for 5 weeks however, I do still get that 'OMG what am I doing' alarmist feeling but it lasts moments and has decreased with HRT. I'm not sure if it's the acceptance of becoming who we are that helps or the HRT interaction with the enorphines (I'm no endocrinologist!) but I have noticed a marked difference.
I am yet to to try this out with my regular partner, but fingers crossed all will be well.
Hope that has been of some reassurance. Best wishes
Amy x