I've seen hundreds of beginner traders fail because of this one mistake:
They commit to an opinion & are unable to admit they were wrong when their opinion is proven wrong.
Whether it's a day trade or swing trade, successful traders have no problem taking a loss & moving on. We understand that a 100% win rate is impossible, so we commit to keeping the losses manageable
Humans are built to be stubborn. Most of us refuse to ask for help, don't apologize when we are clearly wrong, ignore the advice of much smarter people, etc. We don't like admitting what we perceive as 'weakness'.
It actually takes strength to admit when your opinion is wrong. You have to put your ego aside & admit that you were wrong, which is something you have to do a LOT as a trader.
Your market analysis is going to be wrong a good portion of the time. Being able to see that, cut the loss, and move on can be the difference between a small loss and a blown account.
Here's what I tell traders who struggle with this:
DO NOT ENTER A TRADE WITHOUT KNOWING THE INVALIDATION TRIGGER
Make sure you understand what would break your setup. Every setup has an invalidation level. Make sure you know what it is BEFORE YOU ENTER
After that, it's all on you. Stay disciplined & make sure you exit the trade if it invalidates your setup