What's the tool that helped you most in trading?

Stop Run Levels.

Not this crap again.

strong and weak analysis + support and resistance + scalping

Cool, I´ll make $ on your stop.

Cool, let me know when you figure out what a stop is.

The ones I trade, where most retailers sit ...remember the duck?
But anyway, honestly, all the best of luck to you.

These two things are more of a mindset than a "tool", but they helped me a lot in the stress level category:

1. When I stopped trying to get in a trade at the absolute lowest and getting out at the absolute highest. I missed a lot of trades trying to save some money getting in and lost a lot of profits trying to make the most. Now, if I want to be in the trade I get in as low as I can, but I no longer try to eek out every penny. And I still get out at a reason to get out, like resistance or towards the end of the day or such, but I no longer hold just to make a little bit more. I may leave money on the table but I put a lot in my pocket!

2. I used to panic if the trade went against me after I entered, but now I know that a trade has to "breath" and as long as the reason the trade was good in the first place hasn't changed, that's fine. Matter of fact, I often use those times now to add to my position.

Being focused and "in the zone".
Allow your System 1 and System 2 thinking to mesh and work together.
However, don't let System 1 take control at the expense of System 2.

hate to post this... My brain and eyes.

as not all my positions are profitable so good money management is very important to me.
so i can make a profit.
my strategy is based on series.