After playing and watching all kinds of poker over the years, I have seen many players give their money away for many different reasons. This top three list will give you about ninety percent of the probable cause of losing your poker money.....
The number one reason is simply....BAD LUCK!!! Everybody goes through stretches of back luck where they just don't get the cards to fall their way. You have to be able to ride through these streaks until the cards come back to you. If you play smartly, throw away the bad hands...don't force the issue...the less likely bad luck will continue to drive you down. Just don't be foolish and add to your bad luck!! Play smart...if you are in a bad streak..take a break or quit and come back later. If you consistently lose after this, it may be something other than luck such as....
Reason number two.......lack of fundamentals. This is very common. Learning fundamentals is as easy as picking up some books on poker and working on your fundamentals. You can also watch the poker tournaments on tv and listen to the commentators. Most commentators are professional poker players that give you great tips on how and why players are winning at poker. There is an abundance of material out there to enhance your playing skills....use them!!!
My final reason is that you have become too predictable especially if you were once winning against the same players that you are now losing against. They have figured out your tells. Maybe you never bluff. They have learned to throw their cards in when you raise because they now know that you have a good hand because you never bluff. Change things up and become creative...you will see that losing streak stop.
There are many other reasons that you may be losing but if you can control these three to some extent you will see your bankroll start to increase.