This is a number one challenge average person has, when it comes to exercise. Sat thing but true. I know it first hand, because I had those moments as well. To stay motivated myself, I looked for a good workout partner. Because when you had a hard day at work, or hard day at school, or whatever case there was, your partner will make you go to the gym and make you do the work you had planed.
For me personally a smaller, hungry to learn and get bigger guy or I should say workout partner did the trick. When I do workout with the smaller guy, I do push myself harder, to do more and to reach the goal i am after. And this is worked all this time for me. Just put it this way. You know the smaller partner, will do whatever it takes to get as big as you are. He will work harder, he will try to lift more every time he is in the gym, so he will lift heavier. And only that thought would never EVER make you quit. And here is the kicker to it, just think if the smaller workout partner one day will lift more then you do, wow that just straight up embarrassing and this is me being nice. That kind of thinking would never make me quit.
Here is another thing, the study showed, that people were working harder when they worked out with the smaller workout partner, or they thought that smaller guy was easily beatable. And that study showed as well, that the men who are winning, produced more testosterone, then those who were losing. Some bread for thought.
Another good motivation is photos of those people who broke your heart. Another study showed, that people who used images of the people who broke they hearts, were more motivated to do the workouts, and they were working much harder.
By now you know that all those things make you work harder. Because that desire increases your level of adrenaline in your body, and this is the same chemical response that occurs in preparation for physical activity. And you should use any of those motivations to get yourself bigger and stronger.