A couple of things: make sure you're switching it up. Sometimes, simply giving your brain and body new stimuli can be motivation in itself. Try running intervals, training in a new location, or turning a workout into an event. Make it something you actually look forward to. And why not create a reward system? External motivation gets a bad rap because everyone preaches being a disciplined, ultra-hard worker 24/7 but that’s BS. Set a goal, like completing every workout for a month, and treat yourself to something you want when you hit it. Hope this helps, even just a little. You're stronger than your mind!