#1: trying to go too hard, too often (# of HIIT sessions per week). If you're over 30 you should only be doing 2 high intensity days per week, at least if improvement is your goal (vs. just enjoying your sport). Data shows your body won’t recover and adapt to more.
Other sessions during the week should be low intensity, aerobic sessions: not just cycling/spinning, swimming, walking, hiking, etc but doing so at <70% of your lactate threshold (usually just under your VO2 max), i.e. less than 70% of the max power you could maintain for 45-60 minutes. Many times those sessions will leave you wondering: ‘what was the point?’ but they are valuable.
I think people burn themselves out trying to absorb more training than their body can recover/adapt from. They think it’s a failure of personal will rather than just basic physiology.
I’ll offer a second: too little rest/recovery. Kinda related to #1 but different enough to stand on its own.
Too many people get up earlier when they want to improve their strength/fitness/health without going to bed earlier. Your body will benefit from extra rest to adapt/recover better from high intensity training sessions and more aerobic work. Rest is a killer app when combined with HIIT.