@GinPink
Did she thank you when you GAVE it to her (like on the day?)
If she did, then IMO, you are being very precious.
Thank you cards are an outdated concept that don't belong in this century. Many people have better things to do than sit and manually write out 100+ thank you cards, and spend money on them (and the stamps which cost a bloody fortune now anyway!)
I have given gifts for weddings, and births of babies 5 or 6 times in the last 2 years alone, and have been thanked in person. I would find it weird and odd if I got a thank you card too. What a waste of resources, and card, and time, and money!
By the way, it's not their fault that you were daft enough to spend 'more than you could afford' on their wedding money! 
And stewing over it 6 months on is a bit pathetic tbh. A few other posters claim to still be stewing over 'not getting a thank you' card (or even a text!) from a certain person (or couple,) from a long time back. How ludicrous. They must have very little else to worry about in their lives!!!
As @greatduckcookery said, I don't give to expect a thank you card back.
@Pikehau
I wrote thank you cards for most of my 80 guests
Fuck that.
I do admit I left dh a list of “his guests” as I had RSI and my hand ached too much to write them all and 9 years later I am still apologising to them that they never received one from us.
Absolutely ridiculous. 
I have a strong suspicion the bride and groom thanked the OP personally on the day, but she probably will deny it on here.....