You don't have to go through everything in a single day! That would be overwhelming and take too much time. Instead, go through and pick out a few things you know you can part with. Then do it again the next day. Pretty soon, you will be motivated to do more at one time.
Know what makes you comfortable... and own it!
There are many trends and clothing styles that I LOVE... just on other people. For example, I love chambray shirts. The are super versatile and cute. But, no matter how much I try to style them, I just have never felt like I rocked it. I hardly wore the chambray shirt that I owned, but I couldn't get rid of it because of the potential. I finally had to be real with myself and own up to the fact that I don't feel comfortable wearing them.
Ask yourself: "Do I look/feel fabulous in this?"
When something is cute, I feel that I should hangout to it... even if I don't wear it! To help me get over that, I started trying on the clothes then asking myself if I felt and looked fabulous in it. It sounds silly, but this was so helpful!! I had many of clothes that I liked, but that didn't look great on me so out they went. Now that I think about it this tip seems like common sense, but I know many people have the same problem. Only keep clothes that make you feel spectacular, not just OK.
Edit in rounds
What I mean is, initially go through your whole closet and get rid of what you can. Then after a week or so, go through your closet again. The first time some things might make the cut that you wouldn't have the second time around. The first edit I was more lenient with what I kept. After getting more willpower, I was able to go through my closet a second time with a more critical eye and pare down even more.
Do a trial wear
Still have clothes that made the cut or that you are unsure about? Try to reduce even more by giving them a trial wear. I had several things that I still was on the fence about, so I wore them for a day and took mental notes about the item. If there were features I didn't like (like a halter top knotting my hair), I would toss it. Even if the clothes look great, they aren't worth keeping if they become a hassle to wear.
Bonus tip! Get a friend to help
While I didn't have someone sitting in my room helping me decide what to keep or toss, I talked over my plans with a friend. The first senario would've been so much better (and fun!), but just saying your plans out loud to someone keeps you more accountable. Have you ever had a workout partner? Its the same concept, having someone to be there through the process helps you stick to your goals.
I hope these tips help you reduce the size of your closet! Let me know in the comments if you are also trying to minimize what you own.