Kelly,
What about this?
Be sure to plan something like this with a budget firmly in place. Dont EVER approach your mom with a project where you are already panicky about the outcome. (And by the way, Kell, you might sit down and get very still and see why you planned this without a budget. Did you, on some level, purposely sabotage yourself in your conversation with her? Don't laugh--it happens. A lot.)
Then you can go to your mom and say: Mom, I have planned a very simple gathering for D and H. I think it will be lovely and understated and will really touch them. It occurred to me that you might like to do something special and I wondered if you would like to do X. You can do that as big or as little as you like (dancing--the band, lights, etc? Whatever.)
Then leave the outcome to her and practice being relaxed about it.
OR!!!!! Plan what you want to do within your own budget, and tell her thank you so much, but you dont need anything. If she has a chunk of change she wants to spend, ask her to buy something big for the new couple. Whatever she wants. She can even ASK THEM WHAT THEY NEED. Novel thought.
The common denominator in both scenarios, Kell, is that you have already planned carefully what you will do without her help. Then you will be able to enforce a boundary--if you don't have one, you can't enforce it. When you go to your mom, already upset and without funds, you are (on an unconscious level) begging her to dance with you in the same old dance. And, of course, she accepts the invitation. You are both locked into the same roles (you are frazzled and overdrawn, she is powerful and available with cash). If you don't like the dance, you need to stop asking her to join you.
It will mean that you are going to face discomfort on a different level, however, and it may be an enlightening experience for you to face that discomfort head on. There is some level on which you are angry with your mother for her power over you, but you are willing to put up with her powerplays in order to get what you think you need--money for your house or your daughter's events or paying off your bills. The problem is there is a price tag to her money and there always will be. You don't get to decide whether or not she applies the price tag--but you can sure decide to not buy the merchandise.
Smaller house, credit card payment plan, simple wedding reception for D. Those may be harder to swallow than you think. But facing that is where you are going to see some TREMENDOUS growth. I want that for you, in the worst way.
Love,
CB