I think I used to have a similar problem. Especially when I went to casinos-I would by in, triple up, and then I would leave with nothing and I would wonder what happened. So, the next time I go and I triple up again, I had this fear of losing it.
When you build up your stack in cash games, you might feel the urge to play a little looser because you can afford it. There is nothing wrong with that and its exactly what I do. If I've tripled up my buy in, then I have a little more money to play with and I can take those gambles. However, there is a difference between loosening up your play and playing stupid. An example of playing stupid would be to risk half your stack on a gutshot draw. Just because you have the money to call doesnt mean you have to. Heck, just because you are up doesnt mean you have to alter your play at all. You can continue to play exactly the same way if you had tripled up or if you had the initial buy-in.
Do you feel obligated to call because you have a big stack? If you sucked out on someone and you're in a hand with them, do you feel like you should give them a chance to win it back when its a losing situation for you? If so, this could be one of your problems. I used to feel obligated to call a raise with a hand when everyone folded. And its stupid. Everyone who buys in, even you, is saying 'Im risking this money.' So, just because you take money from others, dont feel like you have to give it back.
Try to avoid things like:
-I know you have me beat, but I'll pay you off.
-Im curious to see what you have, so I'll call your bet.
-Betting a medium strength hand on the river...
One thing that has really helped me is to not consider your profit as part of your stack. If you buy in for $25 and you have a $75 stack, that $50 is your winnings thus far and you are only playing with your $25 buy in. Im not saying to ignore you stack completely, instead play as if you dont have a $75 stack. Play as if you still had a $25 stack-so, if you were in a situation where you wouldnt risk your $25 stack, then the same can be said if you were sitting on a $75 stack.
Example: Say you're sitting on $75 and you hold a 10J suited in late position. Its a decent hand to see a flop with and you can make a lot of money if you hit. Except when action gets to you, you're looking at a Raise, Call, Call, Reraise, Call. Of course, the usually correct play would be to fold, but sometimes you might get the mentality of "I can afford it with my big stack" So you make the loose call. However if you looked at it as if you were only sitting on small stack, your mentality might be "Im risking too much, so I fold."
I've seen a lot of people lose a lot of money because they play loose and they thinking "I can afford it." But what they are really saying is "I can afford to play loose and stupid because I have more money." So, try and to avoid thinking like that.
Of course, dont ignore the fact that you have more money. If you hold AA or you're on a monster draw with lots of outs-say open-ended, nut flush draw, and 2 overs to the board, then you're going to want to maximize on your profits..especially if you hold the best hand or your draw hits. If the pot odds given are correct to chase, then do so. You might not hit and lose your stack-and it will happen sometimes, but if you made the correct mathmatical choice, then you really cant fault yourself for it.
No matter if you're up, down, or even, try and play solid smart poker. Avoid getting yourself caught in risky situations with marginal hands, and try to always make the right decisions.
I like the idea Mr. Sticker suggest because I used to do the same thing, except that I play cash games at casinos. So, I dont play a set number of hands, but I play a time limit and a set BR to play with. Usually its 2 buy-ins and 6 hours of playing. I think setting a set amount you can spend and how long you can play is a great idea because it forces you to play smart.
Allowing yourself only 2 buy ins means that you have to play a lot tighter and a lot smarter. Even if I have 100 buy ins and I allow myself that, then I am risking my stack if I dont disipline myself. So, I can either play as long as I can on 1 or 2 buy ins. Or I might lose both buy-ins early and thats it for me. Of course, its so temting to buy back in online because its so easy. If you play at a casino, all you have to do is leave your atm/credit cards at home and bring just enough and you cant get tempted.
The time limit is set because that is when you tell yourself "enough is enough". Unless I am on a monster roll, and I seem to be sitting in the seat of God where every single hand i am dealt is golden-I will follow my own rule. Why set a time limit? Because if you dont-how will you know when to quit? If you're up, you certainly dont want to quit because you're making money. And if you're down, you dont want to quit because you want to give yourself a chance to get even. So, when you do you stop? As soon as I am dealt my cards, I set my phone alarm to go of 6 hours later. When it rings, it doesnt matter if I am up, down, or even, I finish playing that current hand and tell the dealer that I am done. I tell the dealer so they dont deal me in the next hand and I can focus on racking my chips. Also, I dont like like being dealt in after my time limit is up because I might get dealt a hand I want to play...so, i resist temptation and verbally announce that I am finished. Another reason to set a time limit or number of hands played limit is because you can avoid the "one more hand." syndrome. If you're thinking about quitting and you tell yourself "one more hand and Im done." How often does that really happen? Especially online when cards are dealt so fast-its hard to be true to "one more hand." "one more hand" can easily turn into "well, just one more." and then "Seriously, one more" and before you know it you've played another 20 hands. You might have won some money or might've lost some money. And if you've lost, you're probably kicking yourself for not leaving the table when you told yourself you would. At least at casinos, they have to push the pot, gather up the cards, and reshuffle...
ETA: Dont ever feel bad about getting up. There is no rule telling you that you have to sit down for X amount of time or that you have to give the other players a chance to win their money back. If you had a rush early on, tripled up, and then suddenly you find yourself card dead. Or the table is just playing way to loose for your liking, then get up. Call it quits for the day or find another table, but dont feel obligated to sit there and play if you arent able to play your A-Game.