I'm only an IB sophomore, but it sounds like only a few things should fix it.

-Re-read the part of the textbook that you're on ("for fun")
-Do the work as soon as it's assigned
-Practice your writing skills so that you can write a lot about a little without rambling on
-Keep faith and study daily

Junior year is the hardest, but if you try as well as you can, you should be able to make a C or a B.