Over-thinking trades has two drawbacks: firstly, cuts down their potential to make money by limiting the number of trades and secondly when the trigger is pulled they have put so much thought and effort into their trade that they fall in love with it.
They are unwilling to get out of such a trade with an almost-instant scratch trade or an almost-instant small loss. This is why people ride their losses and end up gambling due to their inability to accept so quickly that they were wrong.
Instead of entering into a trade with the confidence that you are right, each trade should instead be entered with the assumption that you are wrong with a willingness to react correctly if indeed you are wrong. As much as you may have built up your reasoning for the trade you just did, you must remember that you don't actually know anything about what is going on and it's alright to be wrong.