You just need to prioritize. I smoked weed daily all throughout my first 3 years of college, and only quit during my 4th year so that I could pass a piss test for med school.
I only smoked after all my work was done, usually at night. You just have to not kid yourself by continuously saying that you'll make up for it later. College really isn't difficult to get through in any major as long as you keep to a schedule. Cut back a bit and set a daily schedule. Figure out exactly what you need to get done every day and only smoke when you're done and you'll be set.