Before my last race, I went into it thinking that my goal was to run a tough race, really feel the pain. I PR'd in what felt like an extremely easy race. So, take from that what you will. Pretty sure it felt "easy" because I was in shape to PR by a lot but I wanted to stay in a pack to draft (10-15 mph wind) so I didn't push as hard. Felt way easier than when I ran 15s slower (1600) the week before.
I highly recommend the book "How bad do you want it". It's all about mental toughness in endurance sports, combining anecdotes with science. Although it's not all running, much of it is, and I enjoyed the non-running stories just as much (cycling, triathlons, and rowing IIRC). It's not really an instructional book but there are tips for running tougher, each chapter sort of has a main takeaway message.