Simply put, tilt. I remember starting off playing online poker tilting like it was nobody's business. I'd load a few bucks and grind the SNG's and MTT's. Sometimes I would hit a big score (for me) and would then transition over to the cash tables or maybe even play bigger SNG's only to lose my whole roll. Then, it was just rinse and repeat. It was a vicious cycle, but I still had those dreams of making it big and ballin at life. Then something happened, I discovered DC/DTB/2+2.
From there on, I constantly worked on controlling my tilt issues. I studied the game more as I felt one of the best ways in controlling tilt was getting a better understanding of how the game worked. It definitely helps knowing that a hand was just a cooler/standard and moving on instead of going crazy and blowing up in the chat box. From there, things slowly started to progress. I went 3+ years without having to reload once and cashed out thousands. Maybe small change for some, but I felt that I had finally started making progress. Then of course everything shuts down and I was left with a decision.
For me, I decided to take a while off and just focus on other things. My career, my relationships, etc. When I did come back to poker, it seemed that I had taken a giant leap backwards. I have made quite a few deposits, nothing huge, grinded up to a few hundred bucks and then boom, gone. I did this time and time again. Finally, I told myself "this is your last deposit, make it count." Now, I'm just a couple buy-ins from moving up to $50NL. Do I still tilt? A little. I have my moments. However, it's the way that I'm trying to handle those moments to prevent me from losing what I have worked so hard for.
So there it is. There's my opinion on the toughest villain at the table.