I used to think that with good motivation, dedication and correct training nothing really could have stopped me and everything would have come to me "easy", work smoothly, with no major interruptions. I thought success (as in training progress) was a straight arrow, slowly but steadily keep pointing up. But training contortion I had to rethink my vision of success and improvement. The arrow, no matter how much motivation, dedication and hard work, NOT always goes straight up, it actually gets stuck, goes down, then up again, etc.
Your body has its own ways to accept changes (=improve), sometimes it listen to you, other times it doesn't. Your body is a very complex mix of bones, tissues, muscles, nerves, ligaments, tendons, organs, fluids operated through more complex systems of chemical reactions, hormones, fed by oxygen and nutrients. All of this strives to work in harmony and keep you safe: that's your body's main job. Often times in training you have to push your body to get out of this safe and comfortable zone and so it gets scared, send all sort of inflammation signals and alarms, gets sore and uncomfortable, but eventually IT ADAPTS, because you (your mind) rule, not the other way around.
So this is why success is not a straight upright arrow, because you are not simply a machine, and your body has to work its ways (get sore, strengthen, adapt, slow down, get inflamed, slow down, heal, strengthen, adapt, get sore...!) to adapt to the changes you want to see. You also need to respect your body and know when it needs a break, because it's really the place you live in for your whole life.
Sometime no matter how motivated, you can't improve, you feel going backward, or stuck, even for weeks. This doesn't mean it's over and you won't improve anymore. The good news is, if you keep your head in the game and don't panic thinking you're broken and will never see progress again, you'll keep improving. Those bad times test your will and determination, as well as your patience: if you get over them, you are a step ahead, mentally and physically.
So bottom line is, success is not an upright arrow, you need to push your body out of its comfort zone, yet be understanding and have patience. Work with it, don't fight it, Things will work out :)