I've suffered from that affliction many times. I used to go backpacking quite often when I was younger, and carried packs much heavier than necessary because of it. In fact, thinking back on all my backpacking and long-distance motorcycle trips, I can't recall a single instance when I needed something that I hadn't taken with me.
In the decades of my travels, I have developed a packing list that I use when deciding what to take along and what to leave at home. During the days and weeks before departure, I use this list to get organized. Tip: Spend the time creating and using a packing list before you leave, so that when it's time to load your gear, you don't have to worry about things at the last minute; just follow your list, knowing you spent the mental energy to think it through and nail down the details beforehand.
This list is meant to be complete, but that doesn't mean I take everything on the list on every trip. In fact, since there is a mix of different items based on season, trip length, and trip type (car camping vs. motorcycle camping) or even which bike I take, there are no circumstances when I would take everything on the list. I use it as a list of ideas of what I might need to take, and scratch off the items that don't apply.
Speaking of scratching, I have a system. For any given trip, I print out a fresh copy of my packing list. I draw a line through items that don't apply. As I go through my home and gather items, I put a dash - in front of them. The day before departure, when I'm packing items into their various containers and stuff sacks and so forth, I draw a small circle next to the dash. It looks something like this:
— Something I've located
O— Something I've packed but isn't on the bike yet
When the item gets onto the bike or into the car, I put a check mark in the circle indicating it's done.
Just before I leave, I review my packing list. Any item without a dash is something I never located. Either I decided I don't need it after all, or I wanted to take it but couldn't find it. If it's something I need to purchase before my next trip, I draw a circle around the entire item to highlight it (a highlighter would probably work best). Any item with a dash but not a circle is something I located but didn't pack. Why not? If I changed my mind that I didn't need it, I'll draw a line through the entire item. Any item with a circle without a check mark is something I located but didn't pack on the bike. Why not? Find it, and get it packed.
Once everything on the list is accounted for—all the circles are checked—I'm ready to leave. During my trip, I make note of anything I took along but didn't use. I also make note of consumable items that are running low. This is often toiletries like toothpaste and soap. For food items, I make notes of things I enjoyed or didn't, and adjust my trip menu accordingly. I also keep track of equipment that is wearing out and needs to be replaced, or equipment that just isn't doing a very good job. In that case, I need to do some research to find a better item (examples of this are sleeping bags and pads, tents, gloves, clothes, etc.)
When reviewing items I took along but didn't need, I review why. If I took rain gear, but had sunny weather, that's not a problem. If I took comfort items, such as headphones I never listened to, or food items I never ate, they are up for consideration to be left behind on future trips. One thing I learned from my backpacking days is that items taken along 'just in case' should always be weighed against their usefulness vs. the likelihood they'll be needed. Can the same function reasonably be accomplished using another item I'm already taking along? Is there something else that serves more than one purpose that can be used instead of the single-purpose item? Anything that only does one thing is given strong consideration for leaving behind. Anything that is only used on very rare occasions, and can't serve some other more common purpose, is considered for leaving behind. You get the point. Three of the most versatile tools you can take along on any trip are a credit card, cash, and a smart phone (with charging cable, of course).
They say you only need two tools in life: WD-40 and duct tape. If something moves but shouldn't, use the duct tape. If something should move, but doesn't, use the WD-40.
You can download a printable PDF version of my packing list here.
I organize my packing list into categories. These are items that I have found useful so far, and my list gets modified after every trip. Obviously, your list would vary based on your needs and preferences.
Riding jacket w/liner
Riding pants w/liner
Convertible summer pants
Wool socks/Liner socks
Casual cotton socks
Shorts / Swim trunks / Swim shirt
Ball cap / Floppy camp hat
BandanasBalaclava / helmet liner
Micro-fiber towel(s)Baby wipes / Hand sanitizer
Digital camera & charger
iPod and iPad or laptop
iPod/iPad charger cable
Journal and pen
Itinerary and coordinates
Air pressure gauge
Bike coverMotor oil
First aid kit
Teflon chain lube
Rubber glovesShop rags
Cold weather gloves
Moderate weather gloves
Warm weather gloves
Spare helmet faceshield
Laundry soapFabric softener sheets
Sleeping bag liner
Mess kit / Aluminum foil
Stove and fuel
Freeze dried meals
Instant coffee / Tea bags
Water bottleSnacks / Candy / Dark chocolate
As a bonus, I also have a to-do list on my packing list as a reminder of things to do before departure.
Change oil and filter
Clean bike windshield
Clean helmet face shield
Check tire pressure
Check weather forecasts
Verify lodging reservations
Charge camera and cell phone batteries
Give itinerary to trusted person
Clean jacket and liner
Wipe inside of luggage
Scrub and grease boots
Load contact info into smartphoneWaterproof riding pants, jacket