Before starting to work on anything, make a strategic plan. What does that mean? Pull out a calendar/agenda/planner, pull out/up all of your classes' syllabi, and mark down everything required of you during the entire semester. You are going to have a difficult time balancing the requirements of all of your classes if you don’t know when things are happening. College is meant to teach personal responsibility along with everything else, and time management falls under this category.
The worst thing you can do is short-change yourself with time. Unfortunately, no one has discovered a way of regaining lost time, so you need to plan and plan well. Only you know how long an activity is going to take. Give yourself the proper amount of time. You will thank yourself later when multiple things start needing your attention simultaneously.
Okay, now we can tackle writing.
I know it might sound stupid, but even if you don’t know what to write, the act of writing itself will help generate ideas. Next time you get stuck, just start writing anything that comes to mind. Describe the room you are in; break down the plot of the text you are working with; you can even just write out that you don’t know what to write. Literally, write out "I don’t know what to write". Just write something. Eventually, your brain will kick back into gear. Be aware, this could take time – even up to an hour – so don’t get discouraged. Just keep writing.