Positive thinking
A healthy mindset is absolutely crucial to actualize gains in ITG. Without it, the motivation to play will quickly wane and gains will be gone as quickly as they were attained.
ITG is definitely a mentally stressful activity. It's hard to pull yourself onto your feet and commit to 90 minutes of uncomfortable physical pain multiple times per week. As such, you have to constantly remind yourself about the benefits you'll receive from playing and the gains you'll make. ITG is one of those rare things in which you get out precisely what you put in.
I try to maintain this by controlling how intense my sessions are, but I have a habit of pushing myself extremely hard and thusly, I start to tire out after a month or two, although I realize fantastic gains during that time.
It's always a distraction
If anything, what usually does me in is some new activity that I discover that I get interested in and then I forget about the last one I was doing. ITG, being a lifestyle, requires an almost total conversion to that life and everything else must fall into place around it. It is difficult to fit a hardcore stamina life into things when it will likely take up all of your time and energy.
Again, positive thinking is vital. By looking forward to the new songs you'll be able to do and the things you'll be able to pass that you couldn't pass before, you can maintain the anticipation and the level of effort needed to reach them. Since my own life has been moving along and new things have been entering it, it's been difficult to maintain the same level of consistency in ITG that I had when I was first getting good, but it's all starting to fit in and I'm confident now that I can keep it up alongside my new hobbies, such as cryptocurrencies and blogging.