Good manners are discipline in public. They cost nothing and say everything.

Holding a door, saying thanks and learning somebody’s name are small acts of self control performed where people can see them. They cost you nothing but the second it takes to think about somebody other than yourself. That is why they read as a signal, because the people with no discipline skip them without noticing.

Pick one you let slide, maybe greeting people properly or thanking whoever handed you your food. Do it deliberately for a week until it stops being a decision. You will notice how differently rooms open up.