Hinakhuynh, I was thinking the other day about how mystery shopping requires bravery, since it's delving into the unknown and putting yourself "out there". Congratulations for taking the first giant step and not giving up!
In my opinion, mystery shopping is essentially acting--with you as the star of the show. As with any great actor, when you're new, you need to rehearse over and over and get in character, before you go on stage. This requires creating a well-developed "backstory" to convincingly explain why your character--in this case--has chosen to look for a mattress and to describe what you're looking for, how much you want to spend, etc.
When you're new at mystery shopping, you don't have the luxury to just wing it. I promise you though, over time, you will get better and more confident--to the point you can comfortably wing it.
As an example, a couple of months ago I did a mattress shop. Since I only sporadically mystery shop, I spent some time developing a convincing backstory and practiced it until I believed it. To make a long story short, the salesman helped me take notes on the different mattresses and when he became visibly frustrated about my choice of adjustable base, it was clear he didn't suspect I was a mystery shopper.
Welcome, my friend!
(heart)
I intend to live forever. So far, so good.