In my case I think it is a subconscious problem and I have had it all my pool playing life. Now I blame it on getting old, but it is still something I still often do. I seem to be able to dig down and get tougher in tournaments than when I gamble.
I love to play so much I don't want the game to end and I may let up subconsciously so I can keep the game going. Knowing how much you love it too, this could also be part of your problem.
I know you are always testing yourself and trying to improve and it might be the games are so close you just can't keep enough concentration going to play at the same level hour after hour. I have been around a long time and almost never get an easy gambling game.
The good news is we will still keep getting action.:lol
I like to think of it this way, sir, and its only a personal opinion. Im going to go with the risk/reward issue here again. In tournaments the risk versus reward is very low. Your reward is high. Your profit will exceed your investment. So incrementally every step you make up your profit increases larger. So you try harder as you get into the money.
This is why guys like Mika, Souquet, etc. concentrate on tournaments. Of course here the odds against making it into the money is higher. So these tournament players concentrate solely on developing their playing skill and dont worry about their gambling skills. They also focus simply on games that have the most profit like 9 ball and 10 ball and one pocket is low on their list.
Cash game players have it different. For every game you win you get one unit in equal return. You lose a game and it reduces your profit by one unit. So pressure on the CG player is much tougher. The more you play the more pressure to build those units. Every time you lose you go back a step. Not so with tournaments. Once youre in the money you only go up. But this is why one pocket is favorable because the gambling knowledge is just as important as the playing skill. And if you dont have the knowledge of the gambler you are not favored.
This is why CG players are the toughest and have to have what you would call extraordinary character. It can make you or break you. I dont have it. Very few do. Im talking about making a living at it here. I have when it comes to my trade but not in gambling.
This is also why older more experienced players dont do cash games well. They dont need it. Many have already established themselves and have a fixed income. Why live with that kind of pressure? Many call them so called "nits" but I disagree. Why does an Efren have to gamble his money when he can make a profit easier in a more comfortable setting. Wouldnt you?
When I say gamble I mean really be put in the situation of losing your bankroll. Not the small stakes of a club player.
Cash game players are of the elite. They are the toughest and they get all my respect. But it is a young mans arena.
Let me leave with one analogic story. John Hennigan (Cornflakes) just beat Dennis Orcullo for $30K at the Derby. But Hennigan won $1.7 million in last years World Series Poker Tournament. That $30K was fun for John. For Dennis, a cash game player, it was life changing.
No, seriously, you are very personable and, after/during a long session you may not bare down as much as early on. I have noticed that many killer players seem to get more vicious (in a good sense) the longer they play. Artie was always said to be that way and wear most folks down. As well, I think a guy has to make sure he takes care of himself during long matches,; have a comfortable place to stand or sit, learn to re-focus each turn at the table. Some concentrate every minute of a long match, great players can " take a break" each few innings without losing the ability to get their mind right back in the game. Bernie, it seems to me, has a great skill of turning it on and off without letting himself out of the game as does Big John and our Ghost as well. 
