Calc I and Calc II were the toughest courses i took in first year probably. that and classical mechanics I (basically just statics) but when i took applied differential equations (diffies) in second year, I had a way better prof and man it makes a HUGE difference. I honestly didnt understand what I had done in Calc I and II until I took diffies. now in fourth year i look back on calc I and II and can safely say i understand everything that course was supposed to teach me, even if I didnt at the time haha
just study man. try to stay caught up, falling behind in a class like that will kill you. work your ass off and i guarantee it will pay off (assuming you will have similar courses in the future) it really is the fundamental stuff so its pretty important, and worth the effort.