But the surprising (or not surprising) thing was that this pattern seemed to repeat itself later on, in a very similar way. I won a lot in a period like a day, but lost it again sooner or later, and offcourse that frustrated me...
Not surprising. What you describe here has a name... variance. You say "I won E300 one day" like that's pure profit. Well, if you're a winning poker player, maybe E100 of that is actual profit, and the other E200 is paying for the times you lost since your last profitable game. You seem too results orientated, which basically means you're reading too much into a small sample size. How much you win in one session is going to tell you nothing about how good you are at poker. Your overall winrate over the course of twenty sessions is more likely to tell you something. Better still, understanding why you won E300 on one night will definitely help. Did you play well, or play badly and get lucky? Were the other guys bad players? How so? Are you likely to play these people again? Did you learn enough about them to know how to exploit further their weaknesses?

Winning is obviously great for confidence, but it can be misplaced confidence if one is led to believe they are better than they are.

Being a professional, I'm not sure it's as fun a lifestyle as some people seem to think. Ask yourself a question... can you keep focus and play your best when you're on a bad run of variance and you're worried about how you're going to pay your rent/mortgage? Can you play under that pressure? How are you going to react when some lucky dipshit hits his flush when you really needed to win that hand to break your bad run of unbelievable bad luck? Are you also motivated enough to spend as much time learning and analysing your past hands as you spend actually playing poker? Are you disciplined when it comes to gambling? Can you step down when your bankroll is disappearing or will you stay at the same level to try and win back your money faster?

Don't get me wrong, I'd love to be able to earn a living playing poker. However, when we're playing to pay the rent, instead of as a casual bit of fun that happens to make a little bit of money, then it's a different world.