Sometimes you just aren't strong enough to pull something off. You might buckle under the pressure or fall short of a target or just get your ass straight up kicked. The important thing to remember is that you aren't a failure or wicked or awful if that happens - you just don't have the capacity to pull that thing off right now.
Even if you bite off more than you can chew, it makes you stronger. After all, how are muscles built? They are torn down before they build themselves up again.
2-part question: