Hi Wendy!
I totally understand how you feel. I also thought it was a pretty difficult challenge at first. In my case, what I did was to consider this not as an exercise to increase productivity, but as a way to learn how to evaluate when I was able to commit to things, and actually making the commitment. For instance, it might happen that a colleague would come to me and ask me when a specific task of the project would be ready. Usually I would answer something like “as soon as possible”. However, during the 7 days of promises week, I took the challenge to really estimate a specific date by which I thought I could deliver, and telling them.
The idea, I think, is not to set yourself for very short deadlines or impossible promises, but to be as honest as possible with yourself. In my case, I sometimes over-estimated the difficulty of some tasks I had committed to, and finished them very early. And a few time, it was a bit more stressing to respect the deadlines I had set. But in the end you still manage to do it, I think you can trust yourself.
So my advice would be to trust yourself. You will find that you are actually good at estimating what you can commit to, and what maybe needs more work / thought before making the commitment.
Congratulations on taking the challenge, you already made the biggest step in this exercise! I wish you a great and successful week, and I look forward to hearing your feedback!