I work a full time job. 5 days a week, 8h a day. I don't have many other responsibilities, so it's not that hard. I would say that gym consumes 2h in one day, including traveling to and from (although my gym is like 1 minute off my route to work) changing before and showering after and that includes 60-75 minute workout. Days when I make plans with friends in the evening are trickier but if I stay disciplined, I make it work. Also, I start work at 7 and leave around 3.30, so I'm home around 6pm after a day of work and gym. And as for being drained after work, my job is mainly thinking (I'm a scientist). I don't know what you do, but I can imagine having a physically demanding job can indeed discourage from the thought of lifting some dumbbells after a whole day.

As for motivation, don't have much more to advise than: you just have to force yourself. I guess sticking with it for a few months and seeing the effects is indirectly motivating. Scientists say that will power itself is like a muscle: the more you exercise it, the more of it you seem to have. Every day is an internal struggle for me, fending off the thought "maybe just today... I can skip gym?" Sometimes I cave, but I do manage to make it 5-6 days a week.