what I always do when I don't want to do work is I tell myself to just work for like 5-10 minutes and then I can stop if I really want to. Usually after 10 minutes though you kind of get in the zone and studying doesn't seem so bad and I usually continue. Also someone told me one time that there is evidence that 30 min studying and a 10 minute break is proven to be the ideal. I have no idea where they got this but it works really well for me, especially when I don't want to study because I find I can usually commit to studying for 30 minutes, but 1 hour or longer seems daunting. Then once studying for 30 minutes isn't so bad you can take a 10 minute break and then talk yourself into studying for another 30 min (and repeat).
if you want to develop a better work ethic use that as your motivation! What motivates me is I always feel really accomplished when I do well on an exam or when I feel like I really know something (lame I know) but just find whatever motivates you because whatever motivates me is not necessarily going to be what works for you. I know a lot of people have said they are motivated by what residency, etc. they want to do, which I think is really good to look ahead but in my own personal experience sometimes it is hard to find motivation in things that are far off, so maybe just focus on doing really well on your next exam and use that as your motivation? This is just from my personal experience though so just try different things and then stick to whatever works best for you!