My [24M] mom [55F] invites my Fiance' [24F] and I for dinner at least once a week. She gets sad if we're unable to attend.

I love my mom dearly but she's always been easy to upset and is quite emotional (she cried when we said we're discussing moving an hour away in the next couple years). My Fiance' and I lead busy lives - most days we get home from work around 5, head to the gym, get home and make dinner, and finally finish cleaning up around 7-8. Take our pup for a walk and then watch a bit of TV before bed.
My mom invites us over for dinner at least once a week. For us it's a little bit much - she's amazing but spending an entire night from 5-9 at her house once a week is a little bit much. Whenever I decline because we're busy she fronts taking it well but my dad has explained it upsets her and she's sad over it. I don't know how to find a middle ground here.
TL;DR: Mom invites us for dinner once a week. We're busy and that's hard to do. She gets sad if we say no. How can we find a middle ground?


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