Forming a concrete trading plan also helps you avoid mistakes you might be currently making while trading; this plan doesn't need to be super complex at all, but if you don't deviate from whatever sort of plan you have and execute the plan you devise properly, you'll remove emotions from the picture and you'll likely be making less mistakes than you were before.
Yes, I'm also a advocate that you don't need to complicate things regarding your trading. Having a entry and exit plan is very important, minimize your risk and mitigate everything. But it should not be too complicated that you yourself are loss along the way.
As for keeping a journal, they're good not only for reviewing what things you might be doing wrong but also for discovering the things that make a trade good. It takes a bit of time to make a trading journal and actively add information into it, but I would highly suggest making one as well if you don't have one already.
And there is where your journal come into the picture, if you write everything down, you can something to look at and analyze what went wrong in your trading, did you jump into it early and didn't look for other indicators? I'm old school, I have a trading journal besides me and always write down things that probably I will need to correct in the future and be 'successful'.