This bears repeating. And repeating. It answers your questions and it's that simple (and that difficult). It's the old Yoggi Berra line: If you don't know where you're going, how will you know when you get there?
You know why BSAM's response is a trading cliche? Because it's an absolute truth, that's why cliches become cliches. You have to already know what you will do in every possible circumstance before you ever undertake the trade. Tension is born out of ignorance and indecision. Good planning is the key to long-term success in almost any endeavor or profession.
Think of a job, any job, that is done well. It will require a plan w/contingencies - lawyer, stockbroker, doctor, olympic athlete, airplane pilot, manager of whatever business... If you're not prepared w/alternatives for whatever circumstances in which you may find yourself you won't know how to deal w/that particular situation in the moment. This doesn't mean you can't be creative and intuitive, a good plan should be adaptable, but successful improvisation is built on a firm foundation that you already know is workable.
Tension also comes from the second-guessing that happens afterwards - "I should have done this or that". When you have a specific plan, even when you lose, which of course you will, you are able to pinpoint what happened and say "Ok, this was an acceptable loss because of such and such." It's not a big deal because losses should be part of any trading plan. You should know your money management stuff inside-out - risk/reward, win/loss, blah blah. There are plenty of threads on this subject. This tells you if you're on plan and how well it's working.
W/a plan you can also analyze to see if any particular aspect that you see going wrong w/in the trade needs to be adjusted - later. A plan is a plan is a plan, but it's not meant to be a static, dead thing, but rather an living organism that evolves as market circumstances and your education and experience dictate. But that refining of the trade always, always comes afterwards, not during, and that evolution and analysis afterwards should also be part of your overall plan.
So what is there to be tense about if you've already worked out every circumstance and your reaction regarding the trade beforehand, during, and afterwards?
Also, generally, and these are cliches also, you should work-out and get good sleep to release overall tensions in your life. These will take away from your trading ablities, as they will any profession or undertaking.
H