Forget motivation. It's useless, you don't need it and searching for it is a distraction. You need discipline. Having discipline in your fitness means that you distinguish what you've set out to do from how you feel about doing it. Throw away how you feel about doing it, it's inconsequential to the task and you will never regret a single workout once you're finished.

Your body can do more than your mind will lead you to believe. Your mind (your inner self who you can't stop talking to, who gets excited, or sad about things) and your body (the vessel you operate in the physical world) are two different things. Your mind will always underestimate the capabilities of your body. Your body will do the things you want it to do, given enough time and...

...Consistency. Your body is efficient. It changes constantly. You don't have a single cell outside of your brain in your body that you had 10 years ago. When you exercise consistently, you body will adapt fast and it'll start to eat up workouts that used to be hard. On the flip-side, if you fail to be consistent, your body will give up it's capabilities in favour of efficiency. Every workout is another brick stacked. Keep stacking bricks.