Talk back to doubts
Whether you're spiriting up a hill or running a marathon, at some point you'll inevitably wonder. "Can I do this?" It's what you think next that matters. I am relentlessly positive in answering doubts, telling myself, "I've done harder things than this before" or 'Theirs is no way I'm quitting." Saying this to myself makes it so much more likely that I will achieve a goal.
This is another way to shut out that nagging, questioning voice in your mind. One way I do it is to obsess over technique. For example, doing a no-hands headstand. I'll think about my spine alignment, whether my core is tight, and how deeply and steadily I'm breathing. If you're fully focused on form, you have no headspace left to think about giving up. The bonus: Your form gets better, too.
Hold a yoga pose five seconds longer than you want to. Eke out a half-dozen more burpees than you said you'd do. Challenging yourself in small ways lets you see how much more you can endure and over time, challenging yourself becomes habit.
Find people who push your limits
Every morning, a half dozen guys come to my house in Malibu to train, and some of them are stronger than me. I thrive on that, and I go harder knowing that those guys will. Seek out people who'll do the same for you.
Do the things you hate
If you avoid something running, weights, yoga it's a sign that's the activity you need to do. Because, let's be honest, you likely skip it because you're bad at it or it feels difficult. Forcing yourself to practice these things doesn't just make you more functionally fit, it builds confidence and turns you into a person who invites challenges instead of one who shuts them out.