Motivation is largely based on maturity.....not saying I am mature and em is not, but in general people that are younger do not have enough maturity to be motivated. I struggle with trying to lift 4 days a week and still keep up running and pt. And to be honest I am not succeeding right now, I haven't run in like 3 weeks, granted I sprained my ankle but my ankle is fine now I just find it hard to go for a run.
I am in the habit of lifting weights 3-4 times a week so it is no longer hard even when I really don't want to lift to just go anyway, I just don't give myself an option. But for running and bodyweight exercises I haven't developed the maturity and habits it takes to go everytime....and I am SO pissed off at myself about it. That's why they say it is very hard for 18 year old guys to make it through any spec ops selection, they just lack the mental maturity to get through a course that is 90% mental. And I'm failing at that right now too so don't worry em you are not alone