As someone who has gone no contact with a loved one, I find that it is hard not to feel nostalgic at times about them. Even when my life is better as it is.
Does Polly ever have those feelings? If so, what is a common trigger of said feelings, and how do they cope?
More than they like admitting.
They mainly miss their Papyrus, Pepper. They never got to give him a proper goodbye, even when they thought they were just going to be stuck in that therapy place.
And thanks to Killer making it look like Polly dusted (instead of ran away like he was suppose to) Pepper is positive Polly is /gone/ gone. And that thought alone gives Polly grief.
Even with Salt(Sans), who they spitefully hope is getting grief for putting them there (and oh is he), they still feel a little bad.
Besides those two, there is also Muffet. Polly doesn't like that they parted on such bad terms, even blames themself for it despite knowing more now.
Spaghetti and spider donuts they try to avoid. But any Classic Sans can trigger them badly. Either via Flight or Freeze. They can't call any of their boys 'Sans' without feeling weird and heavy about it. Rarely, Dust will trigger these responses, and Polly copes by getting overly affectionate. Doting on their boys, or they push themself by cleaning everything and anything.