My biggest issue is being over confident in my abilities. Sometimes I attempt bluffs in the moment that seem good, but when reviewed are pretty terrible. Then in other scenarios I find myself spewing chips in other manners trying to outplay opponents when I simply don't have to. I am my own downfall.
Quickly I've seen a few posts here about why tournament poker is hard and people can't win - the harsh reality is tournament poker is hard and full of variance and most players are not willing to dedicate what it takes to be successful at it. Along with a good fundamental skill set - you need to study your game at least twice a week for several hours per session, using what you can to keep improving and plugging your leaks. To do this you will need to invest in at least 2 programs - poker tracker and icmizer - these will net you money within the first year when utilized properly. Coupled with that is good bankroll management and game selection to net you the best ROI. Chasing the dream of a big score is all well and good but to be successful the real goal is to make money. Now, for me, the biggest problem players face is volume. To help fade normal variance you need to put in proper volume to do so - this means playing a MINIMUM of 100-125 games per month (150+ is even better). I worked a full time job 45-50 hours per week and still was able to meet this and the study requirements every month - if you are dedicated enough, you can do it too - no excuses. You either want to take this seriously, or you do not. The volume requirements are not my personal rule of thumb, its just math.
Hope this is helpful.