Whenever I fall out of my exercise routine, I rebuild it in small chunks. At my peak, I was waking up at 4a, walking to the gym, doing 60+ minutes of weight lifting, 30+ minutes of cardio, then walking back home.

So, when I'm starting from zero again, my first goal is just to walk to the gym and back each day. I don't even go in, I just force myself to get up (probably not quite as early), and go through the motions of walking there and back.
Once I have that down, I start trying to get myself up a little earlier so that I can go in the gym and actually do something. That something should initially still be something easy, so it might just be walking on the treadmill for 15 minutes before heading back home. Every day/week, I try to increase the duration/intensity until I get back to my ideal routine.
Some days I have a serious case of the "I don't wanna"s, and on those days, I tell myself that I just need to walk there, and if once I'm there I still want nothing to do with it, I can leave, but I usually end up staying for most to all of my typical routine.

I find that setting myself small, incremental goals is way more effective than setting one big goal, because with one big goal, if I can't do the whole thing, then I failed, so why do anything at all?

Once I get into the routine, I find that it really helps me in so many ways, and definitely helps my ADHD. I really like morning workouts, but my friend does much better with evening workouts. Try different times of day to see what works best for you.