Hi Wakingfirst
I have some ideas, none of them very reliable
But they occured to me anyway.
This may be the most reliable: We, as partners, may have associated our self-worth with being able to comfort, rescue someone or being useful for something. This is termed a co-dependent thought in some circles (though I don't believe in codependency as an "identity", everyone has more or less similar things to a degree. When we feel anxious or experience self-worthlessness out of nowhere and maybe without being aware (and these feelings are normal too because we are in a difficult situation), the mind may be referring to what it associates with worthiness. Imagining is a coping skill (which BPD and NPD takes to extreme proportions.) Does this happen at certain times to you? What are your feelings before these daydreamings (bored, unhappy, stressed, anxious, empty, hopeful?) Are you full, not hungry, physically well?
My pretty unreliable brain theory
They say love addiction activates certain receptors on the brain. A good feeling flows fast then (like a drug). Similarly, some OCD patients enjoy experiencng crushes because again, the brain somehow focuses on this other feeling (or is obsessed with something else and something happy, a rare experience for someone experiencing severe anxiety). So maybe, for the time being some happy hormones are trying to sort out something in your system.
And my very suspicious psychoanalytical theory but I believe this myself
When we have a problem with accepting some "goodness" from someone, when we unconsciously don't allow ourselves to accept this positive thing - FOO issues, childhood trauma etc- we transfer that good thing and experience in another setting. I once read that some people enjoy imagining their partners with other people in sexual settings because they have a problem with accepting pleasure for themselves, so they are experiencing that pleasure through other characters. (I think there was something like this in a Shakespeare play, probably Merchants of Venice, and there was a psychoanalytical interpretation). Maybe, you don't allow yourself fantasies where you are the one receiving care and love. You say something similar in your post actually "Comforting him the way I would like someone to comfort me" and very positively, you don't seem to be banning pleasure/comfort/love for yourself.
Do you think it would be possible to direct that love to yourself and to others (but not him) at a feelings or maybe daydreaming level?