I try to do things in moderation so not lose track of the efforts to lose weight, but don't kill myself either.
So I will try to have sensible food in the fridge as well as treats (so I put out a bowl of carrot sticks with the dip as well as crisps, and make myself alternate - but I like carrot sticks so that helps).
I tend to be a proper butter and cream kinda gal, but I am getting better about portion sizes over the years. And if I use full fat but vintage or fully mature cheeses, for example, I need a lot less actual cheese for the depth of flavour I want than if I use low-fat cheese which is usually very mild.
Herbs and spices can make a huge difference to dishes. A tomato based curry sauce is lighter than a coconut milk or cream based one, and just as tasty for leftovers.
Don't pretend the world is going to end and buy up the supermarket - get what you need and enough to indulge/treat yourself, but not OVER-indulge. The shops will open again, and as long as everyone has sufficient, that is plenty.
For treats, I try to actually limit the quantity but then ensure they are quality - so I'll get a bag of Leonidas orangettes to nibble on rather than a big tin of Roses costing the same (a decent sized bag costs about ?5-8, roses can be got for 6.75). Or share a big bag of nice crisps (well, DH and DD will scoff a lot quickly!) rather than getting 3for2 on pringles and having them around for days tempting me.
Remember the slimline tonic (although I can't stand diet coke etc), and don't go overboard on alcohol every night. Try to have a glass of sparkling water or something soft in between, or just a plain tonic alternating with G&Ts. Have a glass of water on the table beside your wine glass (put the water on the right so it is what you'll reach for first) and that will slow down those calories (you can still enjoy it, just reducfe the amount). I often buy a few less bottles but of nicer wine for Christmas.
I make sure I get out to enjoy any nice weather with walks, and make time to keep going to the gym/pool (for the endorphins to get enough energy to get through the season as much as anything else).