Get Past the Rules of Chess and Actually Start Improving

You know the rules and you keep losing to the same kinds of mistakes. This covers the three principles that let you make sensible opening moves without memorising lines, the one question to ask on every move until it becomes automatic, two endgame positions that will immediately save drawn games, and a five-minute post-game habit that does the actual work of improvement.

I know how the pieces move and I lose every game. Help me move past beginner level and actually start improving at chess. Cover:
1. The three core principles that underlie most opening moves, so I can make sensible moves without memorising openings
2. The most important thing to think about on every move, and how to build the habit of thinking it
3. How to learn from losses: a simple post-game review process that takes five minutes
4. The endgames worth learning first: two or three fundamental positions that will win games I am currently drawing or losing
5. A realistic improvement path: what separates someone who plays casually forever from someone who actually gets better

Assume I play online occasionally and have about thirty minutes a week to practise deliberately.