Dear OP, I'm sorry that you are experiencing this. Your diagnosis seems to have heightened your sadness and resentment about it. I experience something similar with close family and also feel sad and resentful at times. It's a tough one and I can't pretend to have an easy answer. You're dealing with ingrained habits on both sides. That is to say, you are complicit in this.
Have your children ever seen another person making a huge fuss over you, treating you, buying you a lovely gift or taking you out to the cinema and drinks? Throwing a party for you? Sometimes single parents struggle along valiantly and never ask for or expect anything from their children. The children never view them as a person who needs support or a bit of TLC.
Does this DC pay for siblings or friends? Perhaps s/he is selfish as you've suggested. Or perhaps it's a lack of imagination. S/he might just need some encouragement and role modelling.
Regardless of cause, it's obviously really bugging you! You've been handed a lot of suggestions about dealing with this issue. Some advocate just accepting it. Others suggest you stop going out with this DC, pleading that you are skint. Others yet again suggest humorous banter and direct request.
Ultimately only YOU can decide whether you want to just accept the status quo and work with that: drop your resentment, continue to pay. Or just never eat out with this DC. OR do you want to say/do something about it?
If you choose to say something, it will need to be tailored to your own particular way. No good using crude banter or calling DC out for being tight if that's not the way you normally communicate.
Rather than feeling sad and resentful, perhaps express that you'd love a token of acknowledgement and how much a card and small gift would be appreciated on Mothers' Day. You ADORE peonies - a bunch on your birthday would bring you so much joy - could DC see the way to picking up some on the way over?
Sitting silently waiting has not worked... no good trying that one again.
Here are a few top-of-mind ideas.
Have a complicit friend or relative take you out for a birthday or Mothers' Day tea and pay for you. Ask DC to join you. Make sure complicit friend pays only for you, not for DC. DC witnesses another person treating you.
Your other child is seen by the tight DC giving you a present for your birthday, Mothers' Day or Xmas. In other words, some belated role modelling.
You state directly that you'll be happy to come out for tea and coffee AS LONG AS DC PAYS, as you are on a tight budget this month. Start with an inexpensive venue so that poor tight wad DC is not too shocked or put out.
Let DC know in advance of your birthday or Xmas that you'll be expecting a present. Broad hints might work: I'd love some perfume or a particular gadget. Or that bunch of flowers. Tell DC where it can be bought. In other words, help DC along.
Ask DC to pick up some cakes or pastries or take away on the way to yours for lunch, tea or dinner. DC PAYS. Effusive thanks from you.
In your own way, create scenarios which positively encourage mean DC to put hand in pocket. Speak up for yourself! And if and when DC does make the effort and does come through, show your appreciation clearly so that DC feels proud, rewarded for their effort and more likely to be more considerate in future.
Only you can do this. As the ad says: Cos you're worth it!