I sometimes find that when I have a bad day at work - the kids are crying, it's tough to teach, they aren't engaging in what I'm doing no matter what I try - I get really down on myself. It's hard to be thankful when you're having a bad day.
In church this weekend, Mavis, our priest, told us a story about a family who had just gone through a tough, tough day. At bedtime, when the young child of the family was being tucked in, she asked her mother what should she say to God? Her mother replied that she should be thankful that the day is done and it wasn't any worse.
So that's how we give thanks when it's tough. We be thankful that it's not any worse, and that the day is done.