Oh dear.
Well, you had to go assist at the accident. Some things just have to be done.
Are you in a one party state? If you can record your shops you won't need to rely on your memory so much.
Get a purse you can zip shut. Put any docs you get into the purse and zip it shut before you leave the location. That will preclude anything being blown out of your hand or falling out of your purse.
Take your questionnaire with you (in your car, not into the shop) and the moment you get back to your car, immediately drive around the corner and fill it out, every question, right then and there. Keep a stapler in your car and staple any docs you got to the report so they stay together and you don't mix up shops if you do more than one of the same kind.
Once you do a few shops successfully, you'll find your stride and will have a system of what order to do things in, how to store the documents, how to ensure you don't lose them. Some shoppers immediately photograph their receipts and business cards as soon as they get back to the car just in case they blow away. Not a bad idea to do both --photograph and staple -- just in case.
Always double check your address. Find the address on the building and make double sure you're in the right place. Always review your instructions (before you pull into the parking lot) just before you do the shop to be sure you in the right timeframe, at the right address, and know what you need to time, what you need to ask, and what you need to remember.
If you find you forgot to do something, by filling out the report right away you'll know it before you leave the area. You can go back on some pretext (need to use the restroom, think you dropped your cell phone or left your glasses) to make whatever observation you missed. I've called many a gas station when I've forgotten to ask what hours they are open.
The important thing is to do the report right away so you don't realize six hours later at home that you forgot to get the receptionist's name or description, or forgot to see what brands of cell phones were advertised in the windows, or whether the trash cans at a fast food restaurant were full.
You'll get better at it! You just need to set yourself up for some good successes and get your confidence back.
Time to build a bigger bridge.