It would be pretty fun to look back and think “here’s what I thought at 20, 25, 30, 35, etc”, for two reasons:
1. it would probably be pretty funny
2. seriously, though, I probably actually forget some wisdom over the years. There are probably things that my 20 year old self thinks, that I should still think, but that I let atrophy for whatever reason.

Here’s one for now: start with trust. When you go into a new relationship, assume that everyone’s at least honest and not trying to screw you. (related: never assume malice when it might just be stupidity) Of course, as soon as they break that, bets are off. But in the meantime, start with trust.

An example: landlords. I tend to go into any renting experience assuming the landlord is pretty decent. Sometimes I am proven wrong, but most of the time they are pretty decent. I think some other people have worse experiences with the same landlords. Whether you think your landlord is nice or mean might be kind of self-fulfilling.
(I understand some people have other experiences. there are also huge confounds because I rent market-rate places and I’m a white dude. fair. just my experience here.)