They’re part of the process. You don’t need to fight them, or worry about your wellbeing. Meltdowns-while-training are needed, so you can build the mental toughness required for your race. This is something I learned from my coaches when I trained for my Ironman.
A few weeks ago I was in Austin, running on the Greenbelt and Lady bird lake, when I started crying hysterically for no specific reason on minute 56 of my second consecutive day of three-hour back-2-back runs.
I took this photo when it hit me:
I acknowledged the mental and physical exhaustion, pushed myself through it, and finished my run. To be honest, it was a relief that it happened because it allowed me to welcome with open arms the next training cycle: 15 week until race day!