At this age, there can be a difference between lying and fantasy.
If your daughter makes up a story about school, it can just be imagination.
Now, if she says that Johnny pushed her down and Johnny gets in trouble when he never touched her, that's a different story. A "lie" that hurts or harms someone else is not okay. Ever.
We all walk a fine line, don't we? A friend gets a hair cut and hopes for compliments when you are thinking, "What the heck did you do to yourself?"
We say it looks great and are glad they're happy. Right?
I always told my kids that when it came to me, I might be mad if they told me the truth, but I would be FAR madder if I found out they had lied to me. And, I emphasized that I would ALWAYS find out.
You see, my kids were not good liars. My nephew, God bless him, we all joked amongst ourselves that we were glad he wasn't even smart enough to come up with a good lie. He was smart, don't get me wrong, but "You're not going to believe this, but a bird flew in my window and that's why the stuff on my shelf got knocked down".
Hmmm. There's a screen on the window. No way a bird could get in. Nobody was going to fall for that story. Was it a "lie"? Of course it was. Was he being devious? Not necessarily. He didn't want to admit that he had been goofing off and throwing a ball in his room.
Look, you know when your kid isn't telling the truth. I don't know that tears and punishments are always in order.
My son was fairly young and dragging his feet about getting ready for school. He came to me with a thermometer and said, "You better look at it. According to this I am technically really sick".
I looked at it. I said, "Ummm, according to this, you are technically DEAD, so I advise you to get dressed and, don't let the water get so hot when you run the thermometer under it. Better luck next time."
He was shocked that I knew what he'd done. I just told him that I knew every trick in the book. And, he believed it. It turned out that one of his friends told him about putting the thermometer on a light bulb to make the temperature go up. Like I said, my kids were not good liars. I handled most of it with humor because they never lied about anything super serious or devious. Hey....once you're busted, there's no point in fibbing any longer.
Your daughter shouldn't have to "promise" that she's not lying when you already know she is or she's simply making something up.
You should try to put your trust issues with your mother aside. Your daughter is only 7 and none of that has anything to do with her.
Child psychologists have told me that children often "lie" about things they WISH were true. Your daughter may WISH that she hadn't messed with the Advent calendar and got caught, but she did.
I think it's important to talk to little kids about what they WISH was true and what really is true. Maybe take the emphasis off of the "lying" part.
Whatever you've been doing the past year isn't working.
There's nothing wrong with imagination. It can be great if directed properly.
And, there's nothing wrong with a kid saying, "M., I wish I hadn't done that, but I did." If your trust issues are spilling over onto her, that's not going to change things the way you are hoping.
This is just my opinion and I wish you well.