I dont think logically alot of the time and it always seems its in a big hand where i make a big mistake and lose most of my winnings for the day. How do i make sure i stay focused and keep thinking?

The biggest answer is probably take your time and think about the decision and what makes sense.

How many tables do you play?

4 to 12

Play fewer tables. Become a winning player. Honestly, usually people at lower stakes aren't bluffing. If they're repping a strong hand, they have it a large large large majority of the time.

OK, thinking about this logically, can you suggest one adjustment you might make to your game which would help improve your level of focus/quality of decision making?

EDIT: Failing that, what Nerdrage said.

WAIT!

it somehow leads me to believe that when you say "I make big mistakes" by that you mean you are bluffing or semi-bluffing a lot?. If this is the case try at least telling some believable story and when you get called on 2 streets just don't think : "Oh man I came this far I have to take the pot" or "It's late to give up now"
Learn fundamentals and try to play str8 forward at first...believe me you will learn ALOT about your tendencies and your play

Just be disciplined and remember whats been working. Usually table dynamics change very little and people keep playing the same regardless of if you're up 400BB's. So dont start 3betting Ax suited OTB and defending wtih 95 and crap. Ignore your winnings and focus on effective stack sizes. If everyone else is still sitting with ~100BB's then it's still the same game.

This will come after you'll play more, once you'll get more experienced...

when I was being coached I was asked to drop my number of tables and speak aloud my reason for making a certain decision. Rather than just clicking buttons it forced me to think about and justuify what I was doing. Just basic stuff really.

eg.
"I'm checking the river as worse will not call"
"I'm 3beting to isolate the fish in CO"
"I'm floating in this spot to take it away on turn if checked to me"
"I'm calling down but will give up on any Ace or K turn/river"
"I'm betting for value but will fold to a raise as this guy never bluffs river"

I actually found this to help my thought process quite a bit. After a while you don't need to say it aloud as you'll be saying it in your head. I still spaz out every now and then but find most of my decisions now are at least reasonably thought out.