I recently started studying my sessions and I found I lost most of my money in two situations:
1. Chasing bad odds to the very end, even when implied odds are bad (most of the times OESD or 4-card flushes, and sometimes even gutshots for some reason).
2. Getting a good hand, usually against 1 or 2 players, being last to act, and having him check-calling me to the end.
This hand from my very last session is a good example of what I mean.
Hand converted with online PokerStrategy.com hand converter:
$0.01/$0.02 No-Limit Hold'em (6 handed)
Known players:
[tg]BU$1.31:///////;SB$2.35:///////;BB (Hero)$1.96:///////;MP2$1.74:///////;MP3$3.82:///////;CO$2.58:///////;[/tg]
Preflop: Hero is BB with 9:h, Q:h.
2 folds, CO raises to $0.04, 2 folds, Hero calls $0.02.
Flop: ($0.09) Q:d, 3:d, J:s (2 players)
Hero bets $0.10, CO calls $0.10.
Turn: ($0.29) A:h (2 players)
Hero bets $0.12, CO calls $0.12.
River: ($0.53) 4:h (2 players)
Hero bets $0.16, CO calls $0.16.
Final Pot: $0.85.
Results follow:
Hero shows a pair of queens(9:h Q:h).
CO shows two pairs, queens and jacks(Q:s J:d).
CO wins with two pairs, queens and jacks(Q:s J:d).
I learned my lesson from a while back and have ever since stopped betting for value that much (I would bet 1/2 or 1/3 of the pot with an hand like this, so by the end I would lose too much money to keep this up).
But what am I supposed to do in this situation? I mean I can't not bet, can I? It's not like I'm bluffing, I have top pair with a medium-sized kicker. If he had raised me just once I'd probably throw the hand away.
And also, had I been in first position, most likely this would happen as well. I would bet and he would call to the end.
I read somewhere (can't recall where exactly) that when someone calls you, you should assume they have you beat. Is this a good way of thinking? But then I would check in the Turn, he would bet, and I would fold. This would happen even if he had KJ or Qx, being x = low kicker, like 3 or 4.
So, how should I deal with these guys?
Thanks for answering in advance and good luck at the tables 
EDIT: By the way I knew he didn't have the A because this guy has a very recognizable profile pic and everytime I caught him till now he always raises to 15 (and not the usual 3-bet) everytime he has A pre-flop, unless it's a low, like A2-A8, but then his call on the flop wouldn't be justified