Stop smoking weed for a month. See if that increases your drive in some quantifiable sense (it probably will). If so, stop smoking as a daily habit. Save it for special social occasions and only on weekends.

After you've regulated your cannabis intake, try to get some exercise into your routine, in any way possible. The important part here is to FORM A HABIT and stick with it.

I find that for myself the easiest change was to start cycling to work instead of driving. After about two weeks it started feeling like the most natural thing in the world and now driving appears odd.

After your body is reasonably healthy, if you are still unmotivated then maybe it's time to make a major change to your routine. e.g. job, relationships, location, you probably know which one needs to change. If that change doesn't help rinse and repeat.

Another option is that you're suffering from mild depression, in that case consult with a psychiatrist. Some antidepressants work like magic in matter of days.