What does it mean when someone says “My life was better under Trump”?

Usually it means prices and interest rates were much lower. The inflation rate is still slightly higher than before the pandemic - and of course prices rose a lot before the rate cooled; and interest rates are still well above pre-pandemic levels. So things like housing, cars, education, and any other purchase that requires financing are far more out of reach than before the pandemic.

There's also the very real fact that a lot of "disorder" that voters dislike is much more visible now. More visibile concentrations of unsheltered homeless people, and protection against retail theft (more products behind plastic boxes), generate a very real sense of concern that resonates with voters even though it doesn't show up in the national statistics for reported crimes:

vox.com - Crime violence murder homicide cities downtown

This is a dead end for Democrats. They're not going to rebuild their coalition by trying to convince voters that the insecurity and precariousness they've felt for the past couple of years wasn't real. They're not going to convince voters that have consistently and strongly given Biden poor approval ratings in general (and on the economy in particular) that he actually did a good job.