In all my years of experience, if i had to pick the most common reason why good players go broke or lose money at the tables i would have to say it is due to tilting. Tilt affects your emotional state of mind and negatively impact the decisions you make at the poker table. For the vast majority of us, it is very difficult to completely suppress our emotions during our grind. Especially during long sessions.
So, unless you are some sort of Zen master you will inevitably encounter situations in which you become irritated when playing.
And even though, it is not possible to completely avoid tilt there are ways to mitigate the damage it causes to our state of mind, keep it under control and be at our best when playing. Because; in poker, you are at your best when you are capable of keeping your cool during tough situations and make the correct decision every time. On the contrary, the moment you lose your cool after getting suckout on the river by some "donk" hitting his one outer it´s when tilt appears and that is exactly what you need to avoid.
WHY DO I TILT?
Often, it is due to a profound misunderstanding about short term variance. Remember, variance is what keeps this game profitable for us to play it for the sake of making money (not recreationally). That small taste of victory it´s what keeps recreational players depositing and dumping money on the tables and maintaining a healthy ecosystem. If it wasn´t for variance, poker would stop be beatable and it would be a "reg fest battle" all day long...does it sound fun to you?
HOW DO I KEEP MY COOL?
First and foremost, make peace with variance. Especially short term variance. And always remember, it what keeps those "donks" depositing over and over again. So; without them, there wouldn´t be an online poker site open for you to play on. Also, YOU CAN´T CONTROL IT! So stop acting like it "owes" you something. This is not how a profitable poker player thinks. Secondly, focus on the only think you CAN control.
YOUR DECISION MAKING AT THE TABLES!
Go over your hands, analyze them, take notes, adjust and keep on grinding. This is not an opinion is a statistical fact. The bigger your sample is the smaller the variance will be. So, all there is left is the EV you generate on each decision.