+ the thing is good 
+ entry price 
+ position size 
+ time horizon

you need to nail each of these for a great investment. 

the first part is the highest order thing, but getting the second two wrong can make the right decision a bad outcome. 

horizon is helpful to set up front to ride the waves and assess new info.

This is very true and I think the most common issue is too short of a time horizon. 

If the fundamentals of "the thing is good" haven't changed then you should never allow yourself to be shaken out of the market by having too short a time horizon. 

Patience truly is a virtue.