Dear Ben:
You wrote: “That’s exactly what I’ve been trying to reach myself”, that is, that the physical sensation in your upper back/ body is not an indication of your love or lack of love for your girlfriend, the two are not related. Well, every time you check for that sensation, repeat to yourself that the two are not related. Keep talking sense to yourself. Better yet, take a few deep breaths, relax, and then talk sense to yourself, in a kind way.
This way what makes sense will sink in, as slowly and gradually as it will, over time of practice.
I hope the therapy helps, Merry Christmas and do post any time you’d like.
anita