This is probably a bit defeatist, but if they are subjects you don't like and don't want to do, instead of forcing yourself through it and getting poor grades maybe its better to recognize that its not working and take a risk and change it.
I was in a similar situation last year, I was taking Biology and chem as level because my parents wanted me to do medicine (and I hated both subjects), for the whole year i was miserable, and only scraped a pass for both. I was given the choice of retaking or taking on a whole new a level in my second year. I new how much i would hate retaking so i decided to take on a new subject. It has been extremely hard work but my grades are so much better than if id stayed with those two subjects.
Obviously my experience might be nothing like the situation your in, if your prepared to really work and think you can pull your grades up hard even though the motivation with your subjects is lacking then go for it, its your choice in the end.