I've been doing it for 8 years now since the age of 15 and i've never felt demotivated, not for a long period of time at least. It just felt like a routine, in such a way you have to go for classes/work/video games and eat everyday. When I've felt exhausted after work, I'd just close my eyes, says F* it, and drive to the gym. You can't turn ur car back now, that you're already there. I've made it a habit and a routine out of myself and it's defo something you could try to do. Start off by making your own schedule and choose your On/Rest days. Doesnt have to be 6d/week, 4days are fine so u can have an on/off intervals. And you can put it in a way that working out can help with your performance in sports which does compliments each other in strength/stamina.