Real rough rule of thumb to adjust sag to get to a good starting point. You need a friend, and maybe 2 for the first time.
Put bike on center stand and measure from center of drive unit to a point on the frame above it.Make sure there is NO LOAD on the rear wheel.
Now take it off the center stand and make that same measurement again ... it should be roughly 1" less. (This varies from bike to bike depending on travel, but this works fine for most street bikes) I bounce the bike a few times to get rid of stiction issues. Make sure there is normal weight on the back of the bike and you are measuring from the same two points.
Now sit on the bike, have someone balance you if possible so you can get all of your weight on the bike ... I bounce a few times to make sure there is no stiction issues again. Make the same measurement again ... it should be roughly an additional 3/4" (1 3/4" total from initial measurement). Again, make sure it is the same two points.
Put any rebound and dampening settings in the middle. If the bike is sprung to your weight properly, this would be the normal settings.
Now take it for a ride and see what you think. It sounds like it may be spring too tight for you solo. It also may have worn valving. I would make sure i wrote down all settings right now since you like it 2 up. BTW, you should also make these same measurements two up so you know the proper position to start with 2 up. Realize you will be making a compromise between the two settings (unloaded vs loaded bike)... if the bike has 1 3/4" travel 2 up, the initial setting will be tighter than a 1 up setting once you are done. This measurement is to make sure you are in a good range to start with.