When it comes to fitness & exercise, it's almost inevitable that we'll plateau and / or lose motivation at some point....
Things that I've found help me overcome these challenges:
1. HAVE A WORKOUT PROGRAM... but don't stress if you don't follow it to the letter. Doing 80% of the program for a prolonged period of time is better than 100% for a couple of weeks and then giving up.
2. SCHEDULE YOUR WORKOUTS in the diary as you would other meetings and important events. I've only recently started doing this. Game changer.
3. LISTEN TO YOUR BODY. This is a complex and nuanced point, but overtime you'll learn when to go harder and when to take it easier. A well put-together training program will also help, as it will include rest days and recovery sessions. You absolutely shouldn't finish every workout dripping in sweat having burnt gazillions of calories.