My view is that it all depends on how well you understand why what you are doing should work.
If you understand why your strategy works, be it purely mechanical or wholly discretionary, you should be able to tell yourself how to excecute your edge in the market. Once you have covered all the bases in building your strategy (ot collection of strategies), you already know how to execute your edge - it is just a case of doing it, whether your Gran died / just been mugged on the tube / had a quicky with Brenda from accounts.
Of course, emotions and psychology are important parts of trading - important parts that you just need to trade through. Anyone who says they don't ever get emotional isn't human, just as anyone who says psychology doesn't affect performance is either harded wired already or has never really performed at the edge of their envelope.
One of the things I have written down somewhere is that my profits come from the weak links in the chain - other people [who don't execute]. As soon as I let emotions or psychology play a role in my decision making, I become one of them.
[The irony is that all of this is psychological....]