Credit where credit is due

After looking through some of the recent tournaments I've played and the crucial hands which have cost me a payday, or a better cash, I have come to the conclusion that I am giving players too much credit.
For example, under the 'fresh start' thread My pre-flop raise of 4 x BB was called from the SB with 34o, who then called a pot bet with middle pair on a 732 flop.
When a 4 comes on the turn, I cant see any hand I'm behind to apart from 77 so I'm sure I'm ahead, and end up losing the lot.
Maybe its me being niaive, but I coulodn't envisage anyone calling a raise on level one with 34o in a $100 buy in tournament, nor a pot size bet with middle pair on the flop.
I think Im just going to have to give a lot more thought to what an opponenet may be holding in any particular scenario, not write off some hands on the bass of 'he couldn't have called the pre-flop raise with that'.
I suppose the upside is when a player acts as demonstrated in the 'you called with what' thread. Although it cost me when he hit a 4 outer on the river, the fact that he will call my pre-flop raise with T2, a pot size bet on the flop with bottom pair and eventually get all in with the worst of it will be +EV in the long term, but in the short term its damn frustrating.
I also seem to treat too many players as if they think carefully about what they're doing, which is definately a mistake, for instance if they make a large raise, I think, if they really had the goods, they wouldn't make it so obvious, therefore he must be bluffing, or I assume a check means they have hit the flop and are inducing a raise so I slow down. In actual fact, in most cases, a big raise means, I have a hand, and a check means I've missed the flop.
The overall picture for 2007 is pretty gloomy, down £600 this year, which equates to around 25% of my total online poker profit, it is offset slightly by a £120 profit from playing live this year so around £480 down overall.
The problem seems to be that if I buy in to mid-stakes tournaments ($20-$50 buy in) I run through the bankroll too quickly and end up playing scared, but if I stick to the small stake stuff (<$10 buy in) I have trouble taking it serious and end up playing too loose, bluffing my stack away etc.
I know that just one decent win in a good MTT would put everything right, but to be honest, I cant see where its coming from right now.
Anyway, I'll be positive, try to give every game 100% concentration and hope that the breakthrough comes.