I am actually graduating from my Engineering degree on Tuesday next week and I can say for me it has been an extremely rewarding 4 years. However I really did go through periods where I didn't think I was good enough or smart enough for this line of work but the difference is that in order to do my passion I simply need this degree and that was enough to make me work hard and improve my results.
I think if you are studying something as difficult as engineering you really need to want it because it really is a very tough degree with a super high work load, a lot of commitment and also the ability to work well with others in teams.
So if it is not your passion then I can understand the lack of motivation however the other thing with engineering courses (sure it is the same there) is that the first year or two are very general and more or less cover the basics, it's only the final couple of years where you really branch off into the specifics. So if you really do enjoy the biomedical stuff etc then either stick it out until you start specialising or at least try to get your marks up to a level where you can switch to biomedical engineering which will be more up your lane of interest.