I have won big several times while gambling but I could not keep them. Most of the time when gambling we have to lose because of wrong decisions, it is good for us if we do not get too greedy. Whenever there is extra people during gambling, that is the time when there will be more chances of losing because there is never any extra profit during gambling.
I also had several big wins. I managed to keep a couple of them. I really needed a large sum of money to upgrade old household appliances that had broken down forever. In all other cases, just like you, I lost money again. And all this because of greed, which I could not control.
It seems to me that this is the main reason for all losses in gambling. It is not even discipline during the process itself, but the ability to leave in time with pockets full of money.