Boy everyone definitely takes very different approaches to getting through the mental fatigue. The tricks that work for me seem very different than what everyone else is doing.
I focus on the reason I am training, my next A race, and think about the best way to hit my targets there. If I’m so demotivated that I can’t get on the bike even when thinking about being able to finish that race with my target place or time then I know I’m too worn out. Almost always though, this gets me on the bike.
The pain and exhaustion (both physical and mental) are, for me, a means to an end. The high of finishing well in competition is what I focus on when I’m deep into my 85th minute in the sweet spot and just want to stop, it’s where I live when I’m kitted up and sitting on my couch staring at the bike and thinking how easy it would be to just take a day off.
This, of course, will not work for everyone and is a method that I recognize causes me pain when I don’t perform well at an event I’ve been targeting. I do end up beating myself up from time to time.
Don’t necessarily recommend this for everyone, or even anyone, but it is how I get through the periods where I’m lacking in motivation or my legs are so tired that I’m feeling the fatigue just walking up a flight of stairs
Edit: Couldn’t agree more with @Boombang 's idea above about having someone on the same workout schedule or target event as you. It really helps a ton to have someone to bitch to about how hard the workouts are and to text to brag about how great you felt on a given day. You’ll lift them up and they’ll do the same for you