We all come up against motivational blocks, even the big time pros, so you’re definitely not alone in that department. Expressing your concerns on the TR forum is a great first step.
As other’s have said, really focus on what you have accomplished, and reward yourself for those achievements, rather than focusing on what you’ve left on the table (your tenuous mental state has already been established, you don’t need to keep focusing on that!). Before you get on the bike, figure out the rewards you’re going to give yourself, both during the ride and after (e.g. it might be gummie bears/M&Ms – 1 for each interval completed – and then a glass of delicious chocolate milk for completing the entire workout, etc.).
Try focusing on your physical state/condition while you are doing the workouts; try to separate your mind from your body. Like @chad says, do a mental checklist of all your parts. You might be surprised to find that your body is feeling pretty ok and you can finish the workout. Just keep running through the physical checklist (or focusing on keeping your power on target, like a game) and your mind might not have the space to tell you that you can’t do this.
If you have to, or even want to, take a break – then take a break! It’s not the end of the world. Go for a walk, go to the park, go see a movie and chow down an XL bag of popcorn, whatever you need to do. The bike ain’t going nowhere without you.
And don’t forget that exercise, even activity much more mild than Antelope -5 is really good for mental health.
Quick Googs finds this Time article: Exercise Is Good For Your Mental Health (2018 study)
…exercising for two to six hours a week may be the sweet spot for mental health.
…the researchers found that certain types of exercise were associated with slightly more mental health benefits than others. Team sports led the pack with a 22.3% reduction in mental health burden, followed by cycling (21.6%)
Just keep riding.