Poker is a long-run war of decisions. Two invisible enemies ruin technical players: tilt and poor bankroll management.
Variance: winning well and losing all the same
In the short run, playing perfectly doesn't guarantee winning — luck (variance) rules individual sessions. You can make the right decision and lose the pot. Accepting this is the foundation of mental stability: judge your decisions, not your results.
Tilt
Tilt is playing worse out of emotion (frustration after a bad run, wanting to get it back). Recognizing the signs —playing more hands, chasing, betting out of anger— and stopping is a skill as valuable as reading ranges.
Bankroll
Your bankroll is the money set aside just for playing. The classic rule: have enough 'buy-ins' to survive variance (e.g. 20–30 buy-ins for cash, 100+ for tournaments). Playing stakes too high for your bankroll drives you to ruin even if you're good.