The summer before I started my junior year I went on vacation for a few weeks, and I thought when I got back home and started school again it was going to be an easy year. I was WRONG. Without a doubt, your junior year of college is going to be the hardest.
Freshmen year you're just getting adjusted to college, and you're probably just going with the flow. You may or may not have picked a major, and if you did pick one you probably haven't taken many courses pertaining to your major yet. Professors know you're a freshman, so some take it easy on you.
Now it's sophomore year. You're fully adjusted to college and might have a clear understanding of what field you want to be in post-graduation, and if you don't know everyone tells you that's okay!
Then junior year comes along and you're taking 4+ courses all pertaining to your major and it's hard! Now you actually have to listen to the professor during lecture because these classes actually have relevance and may offer real-world applications to you in your career. If you don't know what you want to do post-graduation, you have to pick something and pick it fast! The dreaded internship search begins, and you apply for a million internships a day. Since you're so close to graduating, if you were a party animal, the partying usually stops because you have 1-2 semesters left until you're thrown into the real world and you just don't have the same energy you did freshmen and sophomore year.
Junior year is the hardest year because there's so much it entails. You have to know who you are, who you want to be, and what you want to do for the rest of your life. Those are such tough questions to be answered in your third year of Undergrad. With faith and hard work, you will get through junior year and will go on to bigger and better things.