To be honest with you, I was in that boat for several years. I knew I wasn't fat but I felt like I was. I still get those days every now and then but not as much anymore. Here's how I personally cope with it.
Stop looking at mirrors for like your whole working day! This means you can look in the mirror in the bathroom and whatnot when you're like brushing your teeth, but aside from essentials like that, just avoid them. This means don't check yourself out in the mirror, window, or wherever you can see your reflection. I don't care if you want to reapply your makeup or whatever. Just don't do it! In fact, probably don't wear makeup for a while if it's really that tempting to go to a mirror to reapply them. If you do happen to come across a mirror or something like that, look away. Because the minute you start looking at yourself, you can't stop checking yourself out, and you start to judge yourself, thinking that you're fat and filling up your heads with negative comments.
Try not to wear too many tight or revealing clothing. This will make you more self-conscious about your body and your weight that you'll start to think on it too much. This will only feed yourself bad thoughts and does no good whatsoever. Instead wear lose and comfortable clothing. When I say this, I don't mean baggy clothes that make you look like a gangster or a hobo. Not all lose attire are considered baggy. Wear something that doesn't accent too much of your body parts like tight stuff but also doesn't hide them like baggy stuff.
Get some people around you, like friends and family, that you have this problem. Ask them for some support to help you cope with this. If you don't do something about this, it can grow into something more serious. That's why if you think you'll need support from others, then find some.
Try to tell yourself good things about yourself. Try to focus on the parts of yourself that you do like and ponder on them instead of on those you don't like. Remind yourself that you are normal and anytime you think "I'm so fat" quickly try to think "No, I am normal!" Also, exercising and eating healthy will help you think more positively and improve your self image. So try doing more of that from now on.
From personal experience, it helped me. I hope that this can help you too.