If the issue is an ultimate lack of interest in medicine, and you wondering if you should do a different major so not even do these courses - talk with an advisor. Get other's perspectives, learn about other options.
If you know you're pursuing medicine or another career which would go from the same major, then you know why you should be motivated.
With the latter situation, your primary problem is your study habits and discipline and not necessarily motivation. Consider the following strategy I use to study:
Study a subject until your mind "wanders". Evidence of this would be needing to reread a paragraph because you weren't taking it in for example. When this happens, take a ~5 minute break and do something else - whether you walk/jog around the building, or browse the internet for 5 minutes. Come back refreshed and engage again.
Initially, you may find yourself taking multiple breaks per hour, but this studying is still more efficient than 6 hours of ineffective glazed over, mindless highlighting. Eventually you'll increase your stamina, and be able to study for longer and longer periods without needing a 5 minute break. Some use the pomodoro technique, but personally I let my mind be the gauge, and easily study well for longer periods than the pomodoro technique calls for, so I prefer this.
This training of your mental stamina would also help with tests like the MCAT, or other long tests. In college I had 3-4+ hour long tests taken during lab sections, and would regularly take breaks on my own as above (as they're not scheduled like the MCAT).